Well...if it were me and the temps were dropping, I'd use hand warmers that are sold in the hunting department of most Walmart, Kmart, etc. These are in packets that you crush, creating a chemical reaction.
These put out quite a bit of heat, but I'd put them in a cloth, not let them be exposed to the cham.
I'd also consider putting him in a small cardboard container with the handwarmers, BUT, I'd really monitor the heat FREQUENTLY to make sure the container doesn't get too hot.
As for humidity, use a hand sprayer. I use a MistKing, but my back-up is the Habba portable and I have plenty of "C" cell batteries o hand. I chose a back-up system, snce we ARE in Hurrocane Season here in FL.
The heating packets may sound extreme and putting youg cham in a small box may stress him out, but I'd rather have him warm than cold!
Gary
Tampa, FL