When I was experimenting... I played with a few of ther thicker bumpers... the ones I listed above just happened to be the perfect height... if you need to drop the height down you can possible screw the clutch stop down a tiny bit with a wrench... I'm not exactly sure whats under the screw hole so I don't know what you'd be forcing the screw into but i dont think a quarter turn or so would hurt anything (the bottom side of the bolt, not the flat side, has a squared head so you can tighten it with an open ended wrench)...
If anyone needs just one bumper I still have the other 7 that came in my package and I can send you 1 or 2 of em... just paypal me $1.50 (including shipping) and I'll drop em in the mail to you. (But mail me first to make sure I still have some left).