Have you used a propane space heater?
My garage/shop is about 400
sq. ft. and my 30,000 Btu heater is not enough on the really cold days, say
below 20, it is not a comfy shop.
I use a Reddy Heater, unvented. While it works well, it has a fan that must
run constantly. After an hour or so, the fan becomes annoying.