I have a reflectex covered windshield shade that folds to the right size to fit under the Heki dome. It doesn't block all the light but enough that I don't get heat buildup inside the camper. I use the bar you pull to open the hatch to hold it against the bottom of the dome. My thinking is to minimize the air space that traps the heat, as opposed to blocking the heat down at the shade which allows the latches, bar, and everything else under the dome to get hot.
I doubt, even without the reflectex, there's going to be enough heat built up to do any damage, but unless I'm camping in cold weather I prefer to keep the heat out of the camper. In cold weather, a dark bedspread under the Heki on a sunny day adds a lot of solar heat.