I have seen some builds that have used small pieces of j-channel or track to hold black out/insulation inserts. I'm leaning towards this on my build.
It fitted properly the tracks do not allow light in/out and the curtains/insulation is easily removed for driving and normal outward appearance. Use a thin piece of closed cell polyethylene foam sandwiched in there for a bit of rigidity while still being flexible. Slip panels in a pocket on the door itself or on ceiling above door so it's handy when needed.
Reflectix/foil-based solutions should be as tight to the glass as possible for best results in reflecting heat, so tracks may not be ideal for all.