I just have one of those little forced air heaters that hooks up to a propane bottle. I don't remember what the BTU's was but last winter in the garage I had that was similar to size of your's I could start it up about a half hour before I wanted to work and open the overhead door about 6 inches for ventilation and when I went back out to work I could take my coat off and be fairly warm yet. It is very portable and the propane seems to last a long time. For me, I didn't want diesel gelling up when I wanted to use it. Also, I noticed that the diesel/kerosene ones seemed to be more expensive for the amount of BTU's compared to a propane one.

One man's opinion. And any heat is better than no heat!!!!

-Jake