I don't really have a good way to do that, but you might want to look into some flexwatt. It is not expensive and it can work with a thermostat. You can build a BIG Brooder box and make the top with hardiboard and put some flexwatt on it. Maybe a fan to circulate the heat better. Just a thought.
My brooders are mad out of wood. I have always kept them cleaned out regularly. I read yesterday, that I should bleach them once a week - what do you all think? Would bleach hurt the next batch of chicks?