I have mine mounted like this: It used to get wet when I washed it so I removed the plastic, laminated the tax disk and use the rubber seal to keep it in place, and binned the plastic disk. Works great!
I have mine fitted near the bottom of my fairing so it can get wet so I photo copy my original on the printer and reprint it each time it gets bad. The original is in my folder!
Dunno why there's so much fuss over this? The Tax Disc holder I have on my bike is waterproof and needs 8 or so Allen Keys or spanners to nick the Disc. It complies with the Law (whether we agree or not is irrelevant) and don't get wet. Simplessss!
Mine is in the inside pocket of my leathers. I've had numerous supposed "waterproof" tax disc holders on my previous bike - all of which turned my tax disc to mush every year. Figured that since you couldn't read it through the misted up glass cover anyway - why bother!