I’d add that, if neither of those options work and you really want to hold onto it, you can probably find a safe deposit box or something in southern NH/VT. When I was living in Mass. I kept my banned mags at a storage facility in Manchester. Nothing illegal about it.

You must have something pretty rare if there are no 10’s availabe. Otherwise Id just skip the headache and buy 10s. If you were to go the modification route, you could block it to 10 rounds and tack weld the base plate on, but it’s still questionable legally and can’t really open it up again easily.