I have a 4x8 shed type coop that works well as a brooder. I think a 4x8 woods would be do-able. 5 foot wide would have a lot of wasted material. Just need to think about how to have one side be a large door, so you can clean it out, etc.
You will probably want a solid top where ever you put it. Other bird will want to get on top.
I have been considering the same thing in my Woods coop and will probably put the brooder area in one of the back corners, under the roost bars.