A question about science/solar heat load/skylight over shower

I will be leaving my FW in Fla. for a portion of the summer. I will leave the vents open, the slides in and disconnect the electricity. I am going to place something over(meaning under) the translucent skylight in the shower to reduce "heat load".

The question I have are there any "cons" to placing a layer of foam, Reflectix or anything else in the cavity? If I place the foil side up I don't THINK that the plastic will be adversely affected but I am not sure.

Has anyone else done this?

I've seen some folks build and place wooden boxes over the sky lights and vents they want protected. Weighted down with a cement block holds them in place. could even put them over the Vent openings as long as a vent is cut into the side of the box.