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This sounds ridiculous, but I just velcroed my toll tag to the back of my registration sticker. Of course I have to remove and replace the velcro whenever I get a new state sticker, but eh. It's not perfect but it doesn't take up any valuable windshield space.
I've done that, but sometimes, the booth didn't read the tag that low.
I just bought a toll pass holder from Auto Barn that has suction cups to stick it to the glass. I Kryloned both the holder (except the suction cups) and the FasTrak pass black, and suctioned it to the rear sunroof. Works great!