I did this mod on our previous popup and hybrid and it helps keep warm/cool air in. Reflectix is bubble wrap with aluminum foil like covering on both sides. It's sold in rolls and comes in different sizes. You can find it at just about any home improvement center. Home Depot link:Search Results for┬*reflictix┬*at The Home Depot
Measure the max height and of a tent end window and total length of all windows so you can determine what size you need. I made a butcher paper template for each window and traced it on the Reflectix before cutting it with an old scissors. I also wrapped the edges with clear packing tape so it wouldn't tear the screening. You can store them under the mattress when not in use. Some people even put a layer under the mattress to help insulate it too. Here's a pic of them installed in our previous hybrid.
Not the best pic so I did a search and found a better one pic for ya. Personally I'd make sure they didn't have any folds in them so they fit better.