I'm using one of those big and tall wire cages with a high sided plastic removable bottom (I had it left over from my ferrets). I removed the levels, filled the bottom with shavings, and used two of those metal clip rings (the kind that screw closed) to attach the heat lamp to the side of the wire at a level that the chicks seem to find comfortable. I plan on adding a level or two back to give them more space (and keep them occupied) and raising the heat lamp as the chicks get more feathers and are able to actually fly around a bit.
I've also got a couple smaller cages (which are similar, but with no levels as they're shorter) that I've used in the past as brooders.
Really, anything can be used as a brooder as long as it can contain the chicks, keep them out of drafts, and a heat lamp can be safely placed above it at a good height to keep your chicks warm.
Also, most heat lamp bulbs are 250 watts, which can be a bit hot for some chicks. I have one but ended up using an 85 watt red tinted flood light for my heat lamp and fastened it about 1/3 of the way up the side of the cage. It stays 80 degrees at floor level, and the chicks seem to enjoy it just fine. Many chicks raising guides recommend you keep your brooder at 90 to 95 degrees the first week, but it's best to go by your chicks. If they don't act too cold (piling on top of each other and staying huddled under the lamp) or too hot (panting and trying to get away from the light), then you're good to go.
