Wiring garage heater to fit a NEMA 14-50 range plug
Your concept makes perfect sense, and electric heaters are cheap.  For instance a 2000W Cadet heater is $50.   Of course that's made for permanent installation, not that that's prevented me from installing one in an afternoon and then uninstalling it the next day. You just have to use correct, legal techniques.  The accessories to install electric heat - breakers and wiring - aren't expensive either. 

Paying for the electricity, on the other hand... 

Anyway, if you want the heater to be cord-and-plug connectd, just change the receptacle to a NEMA 14-30 or 6-30, and put same plug on the heater, and change the breaker to 30A.  Breakers are $9.  You can also just hardwire the heater, but still change the breaker to 30A.