For my brooder I didn't want to have to leave a lightbulb on for heat 24 hours a day. This is the first time I've raised chicks, but it seemed unnatural to keep them in full light day and night.
My solution was to place a space heater below the brooder box, which is just a big 30 gallon plastic bin. I already had built a wood and hardware cloth "cage" for the space heater to safely sit inside, as I used it to heat my three little kids' room and my wife was (rightly) concerned about clothes, blankets, or paper being set on fire.
I used bungee cords to hold plastic lids on the left and right sides of the mesh cage, just to focus the heat upward as much as possible.
The roof is made up of various pieces of scrap wood laid across the top of the bin. I find this to work very well - if it gets a little hot I can pull off a board or two and let it vent. If I want to warm it up quickly I can seal it up with more boards..
Note that I still use the heat lamp during daylight hours. I end up with one end that's nice and warm, and the other is five to ten degrees cooler.
I've run this setup this way for one week, and have no trouble getting to and maintaining target temperatures. I shut off the light overnight, come down in the morning, and find it the same temp as when I left. The space heater is running on a "low" setting, with it's thermostat set quite low as well (3 out of 6).
One other little innovation - I found that if i hung the feeder and waterer from a square dowel I could adjust their heights by simply turning the dowel another quarter turn. The square dowel is held in place to the top edge of the bin with quick clamp.
My speckled sussex ladies... day 7.