I have heard of people using a Mr. Heater Little Buddy with a strapped-in propane bottle, but don't know if their rig was enclosed.
+/- $65. An enclosed cab may raise carbon monoxide and fume issues.
I chose a Li battery heated vest and expect to do the same with socks. We have an old flannel lined sleeping bag (nylon slips around too much) that we tuck under our chins and over our laps. And now we have a 7" reversible fan with in-cab ducts to blow hot air from the engine compartment under the sleeping bag. This works for me, but I wouldn't want it if I was out in severe cold a whole lot.
Don't tell anyone on this Board, but I've been in a Popo Ranger Northstar Edition in hot and cold a few times, and that's pretty nice.
Of course, I kept one hand on the handle in case of fire.