It could be that a little grease is all it takes.
Consider the pressures between that ring and a clean piece of plexiglass; the stiction (static friction) is probably what kept your watchmaker from getting it to go. Once in motion however, the ring ought to keep sliding until it's in place.
It's like seals on a shock absorber: they prevent the shock from being reactive to small bumps, simply because they're so stuck to the damper rods. Hit the rods with a shot of WD-40 and your shocks are magically smoother.
So replicate that by adding some slippery stuff to your crystal, and the ring should slide into place without a lot of drama. Have your watchmaker go for the gusto and cram that thing on in one stroke. Don't let her be a wuss and stop halfway down.
I bet it will surprise both of you.