The problem with other sources is that all but water heat is high enough to ignite wood DUST.
The only other option I can think of would be a construction heater called a "Salamander", but I would place it outside the area and use a piece of duct to transfer the heat inside, where you are working.
As I'm sure you already know; if you can't insulate, you're going to lose a lot of the heat. You might think about blocking off your work area with Visqueen (Plastic sheeting). We used it in a barn we were renovating into a community theatre and it helped.