Given that the Heki is not made from UV or heat resistant materials (sad but true), it would be better to cover it from the outside. Several European camping suppliers sell covers for them. They are cloth, and have elastic around the outside. You can open the hatch from the inside, put it over the plexi dome, then shut it again, all without getting on the roof. I imagine they have to be removed before driving, would probably blow off. That will keep UV off not only the dome ($400+ plus shipping) and hinges ($30 each plus shipping) but also the latch and frame inside.
I wouldn't put anything inside that seals in the heat, reflective or not. The Heki is fragile enough as it is.