Well, I have one 4x4x6 Lowe's play house coop I used once when it was brand new (the babies now live in it all grown up), and just heated it with two 100W bulbs. Two 100W's so incase one went out in the night, the chicks wouldn't get chilled. The play house has a wood floor.
Then I also have two 2x4 foot hutches made from a single sheet of 3/4inch plywood that I put two 100W bulbs in for the same reason above. However, it sits directly on dirt. I prefer this method because then they get exposure to the stuff they will be living on for the rest of their lives. I have it against a 4x8 run that I open the door to at a few weeks old so they can have more space.
I have the lights on a dimmer to adjust temps and take heat away at 6-7 weeks or so. It has been warm enough in there to brood day olds even when there was 8 inches of freak april snow out. I do however keep the lights about 6 inches from the floor. You can see in the snow pic that the side the heat lamp is on, the snow is partially melted down lower than the rest.

Then I also have two 2x4 foot hutches made from a single sheet of 3/4inch plywood that I put two 100W bulbs in for the same reason above. However, it sits directly on dirt. I prefer this method because then they get exposure to the stuff they will be living on for the rest of their lives. I have it against a 4x8 run that I open the door to at a few weeks old so they can have more space.

I have the lights on a dimmer to adjust temps and take heat away at 6-7 weeks or so. It has been warm enough in there to brood day olds even when there was 8 inches of freak april snow out. I do however keep the lights about 6 inches from the floor. You can see in the snow pic that the side the heat lamp is on, the snow is partially melted down lower than the rest.