A 12 gauge shotshell container with slip on cap.
A little sanding of the larger section where the cap slips on is required. This is intentional so that you can get a good, snug fit. A little 100 grit sandpaper and about 5 minutes ought to get it right.

I can get this into the feed tube, and it will chamber when I pump the action, and then eject cleanly on the next pump. But that's a stupid use for it. If you need a dummy round, use my "12 gauge Shotshell no brass" of which this container is a remix.

The better use for this might be to keep small items in your shotshell bandolier or buttstock shell holder. A fishing kit, matches/firesteel, tiny survival kit, medicine?

I may try a screw cap for better security, but this is pretty tight since you customize the fit by sanding. Don't sand more than necessary. A tight fit that you don't have to press too hard to get on is what you want. If too tight, it's really hard to get the cap off. If too loose, it won't stay securely closed.

I would either print at 100% or perhaps use spiral vase (I haven't tried that yet.)