I use a cheap fold up foil-faced cardboard thing....Works for light and
privacy and some heat retention and blockage from sun heat...Folds up and
slips behind my trash can...when it wears out I get another.   I 'fit' it a
little (with scissors) so that it tucks behind the inside mirror and the
sun visors...it actually works pretty good...If it is going to be hot, I
have to park in the shade, my van being dark brown...or go outside during
the day and hang in the shade.