I print fins with a void for a piece of G10 core, as most of my fins are airfoiled, either with a symmetric NACA airfoil if subsonic, or a symmetric Von Karman profile if going more than Mach. I epoxy the G10 core using thru-the-wall construction, and slide on the fin, gluing it with PU glue. That way, even if the printed fin should fall off, the G10 core would act as a fin, as it is about 80% of the area of the printed fin anyway.
I've also played around with the "two halves" idea, adding a couple of layers of wetted 80gr fiberglass between to glue them together, but the epoxy doesn't stick to PETG that well. I may try it with PU glue someday. It does seem fairly stiff with the fiberglass, and good enough for a minimum diameter rocket where you can't use thru-the-wall construction.