Nice job of work!
You probably used the best glue for the task, IMO. I would've probably used epoxy, but who knows which is better. The main concern with any glue would be the disparity of materials and how they react with each other and the glue when subjected to extremes of temperature. (Since the gun probably won't see a snowy cold day in the woods ever again, it's likely a moot point.) May well be why the original plate fell out.
Thinking some more, I thought of using 3M 5200 bedding compound. It's a substance we used as an adhesive/bedding compound when attaching deck fittings, etc. when I worked in the marine industry. It comes in squeeze tubes (in different colors) as well as caulk gun tubes, and I'm here to tell you it is some nasty tenacious stuff and it remains ever so slightly elastic, allowing for movement of the disparate materials.
