We have skylights in our Airsteam, and it came with little form fitting "blankets", for lack of a better name, that you can push up into the opening to keep some of the heat out/in in extreem temps. They are thin aluminum foil cover looking things that could be easily fabricated to fit in other top vents, given you could find the material. I've also seen this type of material used in insulating cars. Just maybe an idea.
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