I tried putting a tiny o-ring under the edge of the cup (in place of the felt washer) and although it worked fine on test every time I went for a drive on a country road the sudden suspension movements would cause it to pop off...
I imagine a build up of hardened road gunk could do the same thing, which is what the felt washers are there for. (To keep dirt out of the joint, but be soft enough not to prevent it tilting)
Regards,
Simon