There is a Pop-top rain cover made in WA. I think that it would insulate
while the top is up by providing an air layer between the canvas and the rain
top.

I have also thought about cutting some us army foam sleeping mats to the
angle of the pop top so that I could put them along the canvas (snugly).

Of course a lot of people just don't pop their top in the winter while
camping.

Todd Thompson
84 westy
Washington DC
too chicken for cold weather camping