We have had a problem with the adhesive on our i-Pass transponders getting soft and letting go or sliding down the windshield in the hot weather lately, so I designed a mount that uses three 45mm (1.75") suction cups to hold it to the windshield. the transponder slides into the mount easily, but is held snugly and won't slide back out.
I printed it in PLA but almost any material could be used.
The suction cups are a very tight fit to get them into the mounting tabs but a little lubrication might help.
The cups I used had a upper diameter of 15mmn a neck 8mm in diameter and 2.5mm wide, and a bottom diameter of 18mm.