Assuming you mean something clear, there's the plastic film that you stick to the molding, and then shrink with a hair dryer. 3M's product seems to work the best, and it's easy to remove in the spring.
It'd be worth trying some of the 3m window film. It is very effective at sealing out drafts and their double sided take is very effective. Use a hairdryer to smooth out the plastic, and it ends up no looking that bad, and you still et light through the windows.
I lived in a really crappy old house in college HORRIBLY maintained single pane wood windows that hadn't seen caulk in years, and the window film made it SO much better. I recall having tried a discount brand one year and concluding that the 3M kits were worth the extra money.
Use foamboard your window is likely rated R1 , the pink or blue is R
5.5 per inch and foilfaced R 7.2". Dont paint a sun exposed piece black, it can melt from trapped radiation and even crack glass. It will make you warmer, just pop out the panel in daytime if you wish. Plastic film may add R 1-2 and helps a bit too.
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