Use the cheap stuff at the dollar store, just tape a couple pieces together to make it large enough. All you need after that is scissors and some time.
If your happy with the fit then you have templates for a more expensive material.
I choose to use refletix and magnets. for the side windows on my E-150. I actually cut them a tad large so I had areas for the magnets to hold. Once happy I glued the magnets to the reflectix I stash them behind the passenger seat along with a standard large window shade for the windshield. I use them for keeping the sun out and sometimes for privacy/security, keep nosey eyes out.