3.5" x 2.875" yes? Mild steel? I really like closed containers which thread together since you get to bore out pockets and make male and female threads. I just made a kind of WW1 grenade but from the video game Team Fortress II with my son (google Ullapool Caber). We did male and female 42mm threads, the latter in a brass ring that got pressed and laser welded into a brass tube with a cap so it makes a secret compartment. He's 14. I told him at some point he could keep condoms in it.
I'm not sure what would be a good use in this size range though, especially since mild steel isn't ideally food safe. Whatever you do, get a fresh black oxide kit e.g. from Caswell, and coat them. They will look great.