A tip for preventing the rear sight from sliding off. Speaking from experience.

Get some crazy glue and put a VERY small bead on the forward edge of the sight, right at the base, in the middle. Let it dry. This should create a raised bump of super hard glue. When you drift it into the slide, the glue will act as a spacer and provide resistance when pushing it in. A mallet and punch may be necessary depending on how thick the dab was. If it is too tight, pull it off and file it down a bit. If it gets too loose again, add some more glue.

Remember to let it dry before sliding it on. The idea is to increase the thickness to provide greater resistance, not to glue it in place.