Harbor Freight Portable garage
I bought the Harbor Freight unit 2 weeks ago when it was on sale (160.00)
I bought a large blue tarp for a "floor" and set the thing up on that. Then I duct taped the overlap of the "floor" to the tent. Then I lined the floor on the inside with heavy black plastic and taped that top the inside.
Now, I'm not heating it, it's just being used to store #1 son's 68 Cougar while I finish the bodywork and paint on my 68 Fastback.
You might want to try one of those electric oil filled radiators. But, beings it's a tarp, it will melt down if things go horribly wrong!!