Steve: I ran a JD 95 for years and the factory heater under the seat would run you out of the cab. Don't any of your combines have heaters??? The cabs on the 45,55,95,105 all used the same heater. It was mounted under the seat. There was a fan behind it to push the heat out on your feet.
On things that did not have a factory heater I have used propane tent heaters with no problems. A cylinder would last a few days and it kept you nice and toasty. You may have a window fogging issue but usually if you run a fan it does not happen much.
Here is a link to one kind of like what I had.