[SkidmoreGarage]

Yeah, that's a bummer. However, it's not the plastic that likely isn't strong enough. It's that it isn't strongly cross-linked from being squirted out like cake frosting The hard part is the design, you have that nailed. I'd suggest getting it printed by a resin based printer through a service.

You could also print it in metal... But that's expensive and likely not worth it.

Just thinking out loud, but why not just use the outer ball design and create a cavity and some engineered flex to accept the old small metal ball. You could then create a "locking" channel for the stem to prevent the outer ball and socket from moving. Then install it back into the new mirror.