Nancy - We had a heated dog mat already, they are pricey, so I understand your hesitation. When I first put our chicks out in the coop, I made them a huddle box. This was when I was still using a heat lamp and I just used a large box, cut out a door big enough for each to get thru and had the heat lamp shine thru the top. It seemed to work for a week, then they outgrew that. Then I switched to the a dog heat mat. I had the mat on the ground of the coop, then I put a brick on top of that, the mat I used needed something on it to release the heat. On both sides of the heater I put 2 large cement blocks and then a straw bale on top of that. I made it where they could escape the heat if they needed to. I also had a straw bale beside this on the floor to block any wind. I have a open air coop, so I covered both open sides with clear plastic and also each side window has a thick moving blanket over it, that I opened up each day so the sunlight could come thru the plastic.
I'm not sure but maybe a huddle box will work with the
brinsea?
When they reached 6 weeks old I took off the plastic on the windows and would open the window blankets each day if the temps were above 50 to let in fresh air, but still closed the windows with the blankets at night. When they were 7 weeks I took out the heat mat and this week I just took apart the straw bales and they just have the roosts now. Of course, I am in NC, not sure if it is colder where you are, this is what I did and I am new at this, but it did work for me.
With each change in the coop, they complain some, but so far they are all healthy and seem to enjoy roosting. Although, I do plan on lowering the roosts, since I don't think I allowed for head clearance when they fly up there at night. They also have a lower roost by the open windows.
If you can figure out a way to slowly introduce them to colder temps, so they have time to adjust gradually, I think they will be okay. Now that mine are fully feathered I am much more relaxed about them, but those 1st weeks are so stressful.