I once brooded some chicks while we were camping in cold spring mountain weather...no kidding...the background is too long a story to post here.
But it was very successful, I kept the chicks comfortable for several weeks until they were old enough to be on their own in the coop when summer came.
I used a black plastic horse tub (about 18 inches high and 18" diameter). I filled the bottom with wood shavings (couple inches). For the first few days, I added paper towels on top of the shavings so they could walk around easier.
A down sleeping bag was wrapped around the entire thing. Down "breathes" and there seemed to be good air circulation, despite the container being totally "cocooned" with down.
I kept this in our tent.
I wrapped a hot rocks or two (about the size of shoes) that I'd gotten warm by placing near the outside fire ring (when the skies weren't drizzling on us!) and when we were having rain, I heated water on my little backpacking stove and filled a hot water bottle, wrapped in a towel, for them.
Necessity is the mother of invention! Good luck!
Maria