Hey dj. I’m actually in the process of building one of these myself. Mines a regular trike with the two wheels in the back though.
I built a wooden box and put the 2 inch thick foam insulation sheet from Home Depot in it, put in a 12 volt pizza bag warmer, and it stays well over 140 degrees inside.
I was planning on setting it up and loading it full of corn dogs, burritos, or boxes of sliced pizza and having people ride them around the neighborhood as delivery bikes. Kinda stopped working on it though.
Anyway, the bike frames themselves are made by a company called husky. Just look up husky trike, I think. I bought a regular trike rather than the reverse trike just because I’m cheap, though.
Hope this helps.