If you really want to be warm fast and cheap, the first step is make a tent in the garage with some poly (similar to a temp spray booth). That greatly reduces the volume to be heated and slows down air exchanges. Throw in a 15A electric forced air heater and while the heater needs to be on all the time when you're working, it will be quite comfortable.
It sounds like PP may have open rafters. If so, he should have lots of ventilation for this propane heaters but also it means getting the garage up to temp isn't really possible as new cold air is being brought in in huge quantities. I've insulated a commercial parking garage before from the bottom. We put up wires perpendicular to framing to keep batts from falling, installed batts, installed poly, then boarded. Technically you shouldn't leave the poly exposed but if money was tight and you were careful with flame, board could be phase two.