Here is a custom insulation job I did that I've been using all winter down to 0F. I used WarmWindow from Joann Fabrics plus blackout fabric, snapped to frame behind walls or velcro to sideliner. Hired a seamstress to do the sewing with extra large sewing machine. Did the snaps myself using the very helpful foot-operated snap press tool at the fabric store.
With the Camco Olympian Wave 6, this setup is very tight and warm.