I like the idea of using self tappers (not that I've ever made an assembly error you understand
). Although with rivets you can just drill them out & re-rivet the offending part. I like the riveted look on my binoscope, it has a couple of kilograms of them.
Cristian, I have a spare focuser if you're interested in having it. It is cheap & basic, but it does the job for these little scopes. PM me if you want it (free).