I have been looking at creating coverings for the windows while sleeping in the van during the winter. Reflectix and fabric coverings are my thoughts in materials. For the rear windows we made cardboard templates and will cut reflectix to size. Then make a fabric covering for the interior face. Fasten with magnets to the well of the windows. Time and effort aplenty. For the two windows with sportsmobile blinds I just cut reflectix to fit inside the space between the window and the blinds. With the blinds down I cant see them. I did the same for the door window and the snapped fabric covering will hide its reflective unfinished appearance. It took ten minutes and I hope will make a difference. Not bad for a lazy winter day. The reflectix roll came from Lowes, special order but worth it.
We used reflectix but no fabric cover. My wife stitched some seam tape around the edges to keep them from getting to beat up. For the front door windows she put a couple of tabs, about four inches long, at the top of each one. She weighted the tabs with flat washers so you can easily flip the tabs over the window and roll up the window to hold them in place. All the others we just wedge into place except she did make a small cover for the vent fan with the hooked part of Velcro tabs stuck to it so we can stick it to the carpeted ceiling. We have only used the vent cover once when it got well below zero' Fahrenheit. Of course they can also help keep the heat out in the summer. She found it too hard to make the windshield cover in one piece so she made it in two pieces with a slight overlap. We don't have a heater but the covers keep it several degrees warmer inside from just our body heat and breathing. We do get a lot of condensation on the windshield even with the covering so if it is below freezing it is advantageous to park facing east to help thaw the windshield but this only works if it is sunny.
The Low-E SSR we carry is very popular with a lot of companies making insulated window covers. It is 3/16" thick closed cell foam with aluminum film skin on both sides. We also use Low-E as radiant barrier in the roof of a van. Reflectix is Ok but the bubbles pop when sewn and they can go flat over time. We also carry Thinsulate TAI1547 which is 5/16" thick and also a good core material. Some use a layer of each. Rip stop nylon is commonly used for the outer layers. Purchase one of our magnetically attached vent covers for an example of how you can make your own window covers.