I have a medium sized rabbit cage from Feeders that I keep the little buggers in --- inside the house until they seem a bit crowded.... then it's out to the coop into a metal dog kennel with a heat light... we remove the plastic bottom on the dog kennel and put in lots of shavings.... seems to have worked great for us....
When they're bigger we just start leaving the door to the kennel open then remove it when they no longer go in....
This of course was during the summer but I've got questions now that it's colder and we went w/o electricity for a week and put the plastic bottom in the kennel and brought the 7 wk old chicks indoors again... now I want to move them out to the coop....