I haven't come up with a solution for Kristi's idea, but to Selective Sensualist's (just moving skin)---
I would think you'd do it like a kneecap (well, maybe this isn't how kneecaps work, but that's how my imagination works).
Have an ultra-firm core, like aluminum (since y'all already have/know aluminum thanks to your Alumina line). Cover it with a soft silicone wrap (molded such that there's maybe 1/4" gap between the aluminum and the silicone), and then fill the 1/4" gap with a fluid silicone (never to be bonded). I don't know if there'd be a way to seal the end after this, or if you'd have to have a double-threaded bolt like in the Alumina line and just "screw on" some balls making a tight seal (and ask people never to unseal it)... but there's my drawing-board sketch. Pass it along, Jenna?