Don't think I've ever seen so many bad answers on a thread. You not only need a level to be level,
but need it to stick level, and stay there. What kind of level are you specifically using (and is it even worth it - most inexpensive small levels aren't very level to begin with). Tapes and goop or
whatever isn't going to help that factor in the least, and probably won't be dimensionally stable over time. It is even a gluable plastic? Or is it metal? How permanent or easy to remove do you want it
to be? Do you have a high-quality different level as a reference, in case you need to bed the small
vial into some kind of epoxy compound, to check the net result?