My shop is block, 20 by 30 with a 12' peak inside. I've insulated the roof with 3" fiberglass (scavanged from a Dillards remodel) then covered it with white ceiling tiles (scavanged from a demo'd Auto Zone). I have a small pot belly stove with a big heat exchanger on the pipe that has an old hang-on A/C blower motor on it. I can literally roast myself out of my shop in about 30 minutes of feeding the stove. I also have a (scavanged of course!) small dual fan behind the stove to blow the heat off the cast iron.
For backup, I (scavanged) my neighbors gas furnace when they upgraded a few years ago. I have it mounted outside next to my door. It blows outside air inside so I can shoot lacquer (I'm a cabinet maker) and not blow up my shop. A little venting and i'm good to go.