I use a type of capton tape that has a double side glue. Works real well. I use it for pbs cubes and mirrors on mm1. I'm sure it would work on flexmounts. In fact I have some coming from Dave to replace the laserman532 mounts on my quad. Now you want to know where to get the tape and what is called.....I'll get back to you I forget.
I use JB weild myself. My friend Jonathon don't really like it but I never had a problem with it. Plus the mirrors don't slide as much when they are drying.