Not much help here, DH has built everything except a brooder so far, but for now I am using a heavy duty cardboard box with a lamp.
I would sugguest that if possible make it big enough to put at least 2 separate compartments in it, in case you have different hatch dates (ages) that need to be brooded.
Well, he didn't want to wait for any advise so I just went out to check on him. He is sooo macho lol. Anyways, its pretty big and octagon. He's great at building so I'm sure it will be great. I didn't want it to be wood but the chicks are coming tomorrow and we already have the wood and I have zero dollars to work with lol.