Portable heaters.

I think the infrared heaters (120V) are the way to go because they are economical; fairly cheap (if you find them on sale, and they don't emit fumes that could irritate you plus damage paint. The casing on the heaters does not get hot so fires and blistered paint, etc. should not be an issue either. It may take a couple of them to heat the area because of lack of insulation. Maybe some old blamnlet and wrapping with plastic may also help.