This Tractor is recycled from a small green house my husband made to keep our dogs out of my raised vegetable gardens.
It is PVC pipe and plastic fencing. My husband is a design engineer, so I had to "reverse engineer" his design. I tried to take photos that would hopefully be helpful for others to easily figure out how to make one like it.
This is NOT suitable for places where you have preditors. It is more for "Backyard" chickens with good fences to keep out any animals that might be able to knock this structure over.
Here is the end view.

Here is the side view.
The pipe that makes up the "ridge of the roof" is threaded on both ends, as are the two long sides where it attaches to the ends.
Here are a couple of photos of that connection.

Close up
By using elbows it has a slanted roof.
It is made up of elbows and T's
The supports are offset a bit, I am not that great at math, but, I am sure if one measures, they can easily figure out how to put the uprights at the right places.
One thing I would do differently would be to not put the horizontal supports in the center of the opening for the door, that makes it more diffecult when you have to go inside the structure.
The door can either open from the side or from the top, or even the bottom if you want.
Here is the hardware
Here is how it works when used on the side.
Here in AZ the plastic fencing is not the best thing for being long lasting. By the end of summer I plan to cover the tractor with regular chicken wire. I like being able to use shade cloth on the top to help shade the ladies from the intense sunlight we have here.
This works well for me, I have a fenced yard and I don't let the dog out there unless I am out in the yard with him.
As you can see, if he wanted to he could easily turn it over or just go through the plastic fencing.