A few years back when we had our own flock still, I wanted something for the young birds to start out in but outdoors, safe from the big birds, safe from predators, safe from family pets, weather, you name it.
I came up with a design that worked out with a 4 x 8 sheet of plywood for the floor, and built it up from there.
It has 1/2" wire front face that is wrapped around bottom, sides and top, so no edges for a creature to pry open and get in from the front.
The sides and center's, including the dividing wall centers were all out of 2"x2" I think. It took a bit of figuring but we got it sorted out to have a 4 ft front, and a 2ish ft back, sloped roof, with the back panels finished flush to the floor, no 2x2, as I wanted the back to be able to come off for ease of cleaning.
We built a solid frame bottom to it, so the back plywood panels had something to secure to, and the whole thing was solid as can be.

This is the basic start of the project. The high side (left) is the front. The lower back on right.

This is the center section. As you can see there is a gap. That is for a 2 pc plywood slider, that can be put in place to have two 4x4 pens, or removed to make it one large pen.
It's all screw nailed together for stability. Nails just shift too much over time.

This is the basic frame once we were ready to start applying outer parts (wire face, plywood sides/roof etc)

Here it is with the wire on the front already. As mentioned above, we made sure to trimi and wrap all edges so the wire had no gaps anywhere. The plywood wall pieces then get applied right over the edge and it assures it will never be torn apart by a fisher or martin, mink or raccoon etc.

Here is it standing on its face. You can see the wire front wraps around (at the bottom side here) and has a small side viewing space too. It's my experience birds can be curious and like to see what is out there, so we offered that little extra for them. It doubles too, for a bit more added side vent if the wind is coming from another direction.

Here it is almost done. We hinged the back doors to the roof so they lift upwards, and can be propped with a sturdy stick or anything while you work inside with the birds for feeding/watering etc.
There are two back doors so you can treat one side at a time. The plywood roof front overhands about a good foot too to keep the worst of rain etc out.

And here it is finished. We built a separate frame stand for it so it could be placed anywhere on the property and be portable, even if it was a heavy thing to move. (two of us could move fairly decently just picking up the ends and walking with it)

And here's the frame for it to stand on.
We simply blocked under the legs if the ground was uneven so the brooder stayed on the level.
The crossers helped keep it from tortioning and twisting out of shape, plus added strength to the entire frame.

And here was our first batch of chicks moved in.
Note the heavy plastic wrapped across the front to keep out the worst of the draft while these guys are small.
The lamp could be secured to a hook in the inside roof, and raised or lowered on a chain.
I was extremely happy with this design and used it for many years!
Mostly in the 4x4 arrangement as I could also use it to isolate a set of roo/2 hens to guarantee blood line and colour/breed when gathering eggs for hatching.
Only thing I found difficult to get by was with full grown birds. They'd occassionally see the big open space around me when I opened the door at the back and in suprise, bolt for freedom.
For this reason alone, I'd consider a different type of rear door, perhaps a mid fold half way across, that still fully removes for cleaning.
Either way, this could fit inside my big coop if in dead of winter, or be safe snug and warm outside, surrounded by some straw bales and sufficient heat lamps for wintertime use.
I live in Eastern Ontario in Canada and winters can go below -20c or colder at times, so mostly, our winters were time to pause, rest and regroup. The chickens got a break from laying and were allowed to enjoy leisure and different food to get ready for spring laying again.
chicks were went we wanted them, so that was easy to plan.
Anyways, thought I'd share my brooder design with you all.
Mostly made from recycled wood as our house was brand new, but not too crazy on the cost basis if building from 'scratch' so to speak.

Jenny
I came up with a design that worked out with a 4 x 8 sheet of plywood for the floor, and built it up from there.
It has 1/2" wire front face that is wrapped around bottom, sides and top, so no edges for a creature to pry open and get in from the front.
The sides and center's, including the dividing wall centers were all out of 2"x2" I think. It took a bit of figuring but we got it sorted out to have a 4 ft front, and a 2ish ft back, sloped roof, with the back panels finished flush to the floor, no 2x2, as I wanted the back to be able to come off for ease of cleaning.
We built a solid frame bottom to it, so the back plywood panels had something to secure to, and the whole thing was solid as can be.
This is the basic start of the project. The high side (left) is the front. The lower back on right.
This is the center section. As you can see there is a gap. That is for a 2 pc plywood slider, that can be put in place to have two 4x4 pens, or removed to make it one large pen.
It's all screw nailed together for stability. Nails just shift too much over time.
This is the basic frame once we were ready to start applying outer parts (wire face, plywood sides/roof etc)
Here it is with the wire on the front already. As mentioned above, we made sure to trimi and wrap all edges so the wire had no gaps anywhere. The plywood wall pieces then get applied right over the edge and it assures it will never be torn apart by a fisher or martin, mink or raccoon etc.
Here is it standing on its face. You can see the wire front wraps around (at the bottom side here) and has a small side viewing space too. It's my experience birds can be curious and like to see what is out there, so we offered that little extra for them. It doubles too, for a bit more added side vent if the wind is coming from another direction.
Here it is almost done. We hinged the back doors to the roof so they lift upwards, and can be propped with a sturdy stick or anything while you work inside with the birds for feeding/watering etc.
There are two back doors so you can treat one side at a time. The plywood roof front overhands about a good foot too to keep the worst of rain etc out.
And here it is finished. We built a separate frame stand for it so it could be placed anywhere on the property and be portable, even if it was a heavy thing to move. (two of us could move fairly decently just picking up the ends and walking with it)
And here's the frame for it to stand on.
We simply blocked under the legs if the ground was uneven so the brooder stayed on the level.
The crossers helped keep it from tortioning and twisting out of shape, plus added strength to the entire frame.
And here was our first batch of chicks moved in.
Note the heavy plastic wrapped across the front to keep out the worst of the draft while these guys are small.
The lamp could be secured to a hook in the inside roof, and raised or lowered on a chain.
I was extremely happy with this design and used it for many years!
Mostly in the 4x4 arrangement as I could also use it to isolate a set of roo/2 hens to guarantee blood line and colour/breed when gathering eggs for hatching.
Only thing I found difficult to get by was with full grown birds. They'd occassionally see the big open space around me when I opened the door at the back and in suprise, bolt for freedom.
For this reason alone, I'd consider a different type of rear door, perhaps a mid fold half way across, that still fully removes for cleaning.
Either way, this could fit inside my big coop if in dead of winter, or be safe snug and warm outside, surrounded by some straw bales and sufficient heat lamps for wintertime use.
I live in Eastern Ontario in Canada and winters can go below -20c or colder at times, so mostly, our winters were time to pause, rest and regroup. The chickens got a break from laying and were allowed to enjoy leisure and different food to get ready for spring laying again.
chicks were went we wanted them, so that was easy to plan.
Anyways, thought I'd share my brooder design with you all.
Mostly made from recycled wood as our house was brand new, but not too crazy on the cost basis if building from 'scratch' so to speak.

Jenny