What size?
There are multiple options available.
First would to build a sump and add heaters to it, I have 2 400W heaters for my 240
Second is solar. Build a solar panel to go in a window. They are simple to build for heating. I built one when I was in High School in Iowa to heat our classroom. My parents used to as supplemental heating in their master bedroom for many years after. Don't know what has happened to it since. Just build a box of 1x4 and plywood. Plexiglas face with corrugated tin roofing under that painted black. Under the tin put in tubing coiled and connect inline with filter. Mine was just forced air but it is amasing how much heat you can pack into a small area.