I always end up planting thing shortly before the cold weather sets in, due to purchasing plants at end-of-season sales. And I have a few plants in pots on the deck I want to make sure survive the next few days. So, i use the bath mat method on some, and cover others with anything handy...sheets and blankets or groundcloth. So far so good
Plant protection below freezing
I too always seem to have things in pots I need to keep alive over the winter. Here is a photo of my little polycarbonate cold frame I bought about 20 years ago. The plastic has gotten a bit cloudy. The nifty automatic opener device still works perfectly. It has a "bimetallic" piece of two metals stuck together, that shrink and expand at different temperatures, so it bends upwards when it gets to about 72 degrees, then closes when it cools off. It has just enough strength for the light plastic cover. No batteries or electricity or timers. Genius!
I put my little plants in a very protected corner with a fence and overgrown hedges on the south and west, where the weather comes from. You can see a few bigger pots that would not fit, but they survived. You can see the Japanese hollies are growing up out of it.
That is really cool!! I'll have to keep my eyes open for something similar at garage sales.