Captain Obvious here: Safety. Some of the modern catalytic propane heaters are rated to be safe (carbon monoxide) for inside enclosures. Among them are the Mr Heater products. Do read the product info.These heaters come from fairly powerful,with fan,to small 3500 btu versions that screw onto a propane cylinder ,take no batteries,and might be just enough.
Some of the older ,lower tech "blind heaters" would insidiously fill a goose pit or blind with carbon monoxide. Folks died that way.
An option that might fit for a few blind sessions are the "hot packs" available at the pharmacy for use as a local "heating pad" for lower back pain,etc.
Wrapping one of those around the kidney area can work well.Do read the instructions and warnings.
Apparently they CAN get TOO hot. (Per the warnings) .
I actually had a Dr prescribe them too me for a back injury. I saw potential for ice fishing.
They aren't cheap,but neither is an $80 propane heater. One hot pad is about the price of a 1 lb propane bottle. Its an option. Its not hard to buy and try first.
Of course,the"Hot Hands" packets inside gloves can work well.,too.