Sporting goods stores carry small red & black heaters by "Mr. Heater". These put out insignificantly small levels of carbon monoxide. There are 2 models, a single burner & a double burner.
A single burner unit (8,000 btu?) doesn't adequately heat my insulated attached 22X36 garage, located 30 mi. SE of Portland at 1,200' elevation. A -9 builder in Milwaukie, OR with a small attached garage has a double burner unit & says he doesn't have to fire up more than one burner on his.
I also have a 35,000 BTU unit that produces unsafe levels of CO & stinks. I fire it up for 20-30 minutes before I go out to work, then turn it off. Last week with outside temps at 39, inside the garage at 49, it went to 54 in about 30 minutes. 54 was OK while working & if I needed I could have fired up the small unit to maintain the temp.
The single burner unit frequently goes on sale at GI Joe's for about $69, but you still have to buy a tanks at Costco ($22) and a hose kit. Haven't seen the double burner unit on sale.
Hope this helps.
Richard Scott
RV-9A Wings
EAA Ch. 902, Mulino