I used
this, in a 2" thick variety because that's what I had laying around.
One side window in our over-cab bed is the emergency escape window, so it doesn't have a storm window (all other windows do). I cut the pink foam board to size and friction fit it behind the valance, pushing the mini-blinds up with it. I didn't have a single piece large enough to cover the entire window, so I pieced it together using a few toothpicks, pushed in at the seam at an angle.
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Very easy to remove and replace. I'm not sure about the Material Safety Data Sheet on this material, or what type of off-gassing it may have. This was an in promptitude improvement while it was 9 degrees outside, but I used it a few more nights. Warmer and no condensation.
If I had stuck a few more toothpicks in, I coulda had a convenient remote control holder.