Those liquid-filled electric heaters that look like radiators are safe and do the job - if you can find a power source.

Finding a power source ?
Have a nice long, heavy-duty extension cord (on a cheap cord reel to keep it from getting tangled when you gather it up)
and one of those adapters that let you turn a light-bulb socket into a power outlet..

you're warm, and no headache in the morning from the fumes!