The link you posted does not go to Heki but to Waeco.
But if I remember reading my manual on it states you can close the shade 2/3 of the way closed to help with heat but not to close it complete. So close the shade 2//3 and have the screen over the other 1/3. I know I sure would not want to put any reflective material or foam or another things in the vent to trap the heat in it. The only way to do this is something on the outside and not let the sun heat it up. Put something in it and I'm sure you are going to overheat it and cause it to crack. Then you well be replacing that. I know I don't every want another one on any camper that I own. Mine came standard on it and if I were to buy another unit that would be one of the things it would not have.
This is IMO
On Edit: Here is the
Link to instructions for it. Look at the safety instructions and it well tell you not to close the shade more than 2/3 during the day