The trick to the 5 minute epoxy is to mix plenty of it... if you only try to mix enough to do a mirror it surely will not cure... i mix about a half dollar amount whether im gluing 1 mirror or ten. I use the loctite 5 minute epoxy from any hardware store.
A jig is very helpful. From personal experence and using custom test patterns Bill made, we have found that the mirror not being axially aligned with the shaft even by 0.002" can cause problems in the projection in terms of horizontal/vertical skewed lines, and being off center excites resonances easily noticable in certain test patterns..
Making a mirror mounting jig is certainly the way to go. That is how we mount all the mirrors on the C506 and Saturn scanners at Pangolin with very few and far between issues... it can take a while to get them set perfectly under the microscope depending on mirror size but once they are set, they're worth their weight in gold and we can mount 5-8 mirrors at a time with one batch of quick cure epoxy.
Sincerely,
Ryan Smith
ScannerMAX Mechanical Engineer
ryan {at} scannermax.com