this works good:

24" piece of black stove pipe 6 or 8inch.....make cuts on one end so that you can flare it out.

12" square piece of plywood with a 1/2" hole drilled in the center.

Mount one of those porcelain light fixtures in the middle of the plywood routing the cord through the hole.

fasten the "chimney" over the fixture to the plywood.

Use a 60 or 75w light bulb.

I use this on my boat at the marina. I have two of them on board and they work beautifully. If a little more heat is needed I sometimes put a clay planter pot on top of the pipe.

Covering your stuff with plastic is probably the worst ting you can do as far as the mildew.