I had this problem until I Ceramic tinted the back window which helped but pricey, the suctions worked ok but as you probably are encountering as the sun shifts given the bigger window sun leaks from angles etc.. another solution we pondered is using the rain cover for our stroller which didnt fit perfect, but there may be a similar solution to cover the car seat when baby sleeping with something breathable, rather than trying to cover the window itself..
PNWMisty, for one OP wasn't asking for your opinion whether it was an issue, it was clearly an issue to not only them, but ATLEV and myself (at one point) .. if you aren't being helpful why post? OP was not asking for peoples opinions, they stated theirs and asked for solutions... Second, OP said they tried the amazon suction cups, if you weren't so focused on rebutting you may have saw that,and Third, you live in washington, you sure you know what bright sun looks/feels like? It's 64 right now for you and feels like 105 for me and probably 100+ for atlanta (OP) .... move on please...but thanks for bumping this post so I could find it