Thanks, Chip. Where in MA are you? I'm in Andover.
I tried a 1/16" thick o-ring, but even that is too thick. I ordered a 1mm thick o-ring (27mm OD / 25mm ID) and we'll see how that works. However, as a low-tech stop gap, I wrapped a thin line of tape around part of the finder to eliminate play at the front. I just did an alignment, and we'll see how well it lasts!
The stock finder had a black, curled, tube-shaped strip of plastic sheet on the finder body that fit just inside the finder mount. IIRC the alignment screws made contact with that plastic strip. Your tape solution sounds like it is close to how the stock finder was configured, except that the plastic strip was not stuck onto the finder.