A punch is what I've always used. You just need to upset the metal around the punch mark into a high enough crater wall to add some thickness. That said, I've only done it where the fit was good to start with, so squeezing the crater wall is what kept the sight from moving. If the gap is serious, it sounds like either the sight dovetail is off or, more likely, there was some slop in the dovetail cut giving it too much width. That will mean that even with the punch upsets, you will have a crack between the bottom of the sight and the slide. You'll want to fill that to keep water out or use a rust inhibitor to fill it. An excellent rust inhibitor would be Lee Liquid Alox cast bullet lube. It is actually the same stuff Zebart uses to inhibit rust in cars. if you get the gap full of it, it should be pretty well protected, though it will soften in mineral spirits and may soften when exposed to some oils.
Another approach would be to shim the top front of the sight dovetail to push the whole sight down hard into the dovetail, at which point the crater walls of a single punch mark in the middle should serve as a friction keeper. However, use a feeler gauge first to learn height of the gap to see if that would lower your POI unacceptably.
If the punch crater walls will hang onto the sight well enough, once I was sure of the horizontal location of the sight I would just use JB QuickWeld as a gap filler to keep water out. The quick epoxy doesn't really stick all that well so it should be removable. You could even wax the sight and slide cut intentionally to prevent bonding so you can change the sight position more easily afterward.
If you want to use a Loctite type product, download their product application and look at the Retaining products (pdf page 6, their pages 10 & 11, here
). Find one that's designed to fill gaps the size of the one your feeler gauge measured. These products only harden in the absence of air, and if you use one that's too thin for the gap, the outer edges may never set up or you may have to use an activator. A product of theirs that I have used successfully on guns is the 660 Quick Metal, which has an aluminum filler and will fill gaps up to 0.020". An advantage to this approach is it will take several hours to set, so that if you did the sight insertion at the range, you could adjust it before the stuff set up.