This is a big question for me at the moment too.
I am trying to devise a plan to be able to brood batches of 100+ chicks, without electricity.
I'm thinking of making some sort of modified hood for the Little Buddy propane heater.
https://www.mrheater.com/little-buddy-heater.html
That's where the trickiness begins. The Little Buddy puts out more than enough BTUs, at an economical enough rate of fuel consumption. It's just a matter of containing the heat without blowing up the heater 😜
Anybody tinker with something like this before?