you just need to get one that's large enough to fit right to the bottom of the screen and all the way up to the vaniy mirror/sun blinds, and rigid enough so you can just flip those down to hold the card in place and it doesn't fold in half.... no elastics, no suckers, etc...
I got a good quality one when I was in holdiay in Cape Town about 7 years ago (for about £2) and it does the job fine: looks like it's cardboard (i.e. has a sort of honeycomb in the middle) but made from a fairly firm plastic and covered in reflective foil.