No matter what sort of glue you use, the cross will likely come off someday. I affix ornamentation to guitars a lot. What I would do (and this is just me) is to use self tapping screws. They wouldn't need to be very long. I would use the ring at the top for one, and drill a hole close to the bottom for the lower one. Orient the cross as you want it, then drill into the
mod through the top hole. Then drive the screw until the cross is tight enough to stay in place, but that it will still move a bit. Position the bottom hole where you want it, then repeat. Now affix both screws, but not so tight that they won't come out easily.
If the metal of the
mod is hard enough, and the metal of the cross is at least a bit softer, then you can use the mod itself as a mandrel. Bend the cross to fit. A hammer may be needed, but try it with your hands first. The points of the cross will probably not be flush to the mod, so remove the cross, and bend the points until you have a nice fit.
Before you screw it back down, you could put a little epoxy inside each hole, then screw it back down. Work a bit quickly. Give it enough time for the epoxy to harden, and then I hope that it's where you want it, because it ain't coming off.
Look at Tamiya model paints. They have quite a selection of military colors. Have fun!
