We used reflectix. Although light weight, it’s good insulation. I had a wide piece and punched snap caps onto it to fit the existing snaps. When it was folded it fit under our jackknife couch. I had cut reflectix for all the windows and vent panels too. Mostly just held by the curtains or shades but with Velcro (that eventually fell off) on the vents. I left the pieces when we sold the rig. Our newer Winnie came with a blanket type material that uses snaps in the same places.
Reflectix BP48010 Double Pack Insulation, 48 in. x 10 ft https://www.amazon.com/dp/B000BPAULS/
