Fastrak Flex toll tags come with adhesive hook-and-loop tape to mount them to the windshield.  Eventually the glue gives out, leaving a sticky mess to clean off of the glass.  I decided I wanted to use a suction cup instead.

Print in PETG or other material that holds it shape at higher temperatures than PLA.  A bright color is probably better than black.  I used PrusaSlicer's 0.30mm DRAFT print settings with an additional perimeter.

This holder prints flat, folds up and snaps together.  It doesn't hurt to add a little cyanoacrylate glue along the seam.  The keyhole slot at the top slips over the nub of a suction cup.  The front opening allows the driver to adjust the passenger count setting.  The back opening reveals the Fastrak logo.  The raised text inside creates friction to hold the pass tightly in place.

I purchased some cheap 30mm suction cups online (they're sold as replacements for holding radar detectors to the windshield).  But it fits many household suction cups, so you may not need to buy any at all if you have some lying around.

The suction cups work perfectly on clean glass but will not stick at all near the very top of the windshield if there are dots painted there to shade the driver's eyes from bright sky.  I found places to mount it in two different cars such that it's behind the rearview mirror from the driver's point of view.

For heavily sloped windshields, the holder will dangle from the suction cup, creating stress that, combined with heat that sometimes builds up in a car, can deform the holder.  The photos show a previous iteration of the design.  The current iteration is stronger where it matters, so I expect it will be fine, but I haven't had this iteration installed for very long.

Public domain, so hack away without worries--OpenSCAD source file is included.  No attribution required, but if you come up with an improved version, I'd love to see it.