I was thinking about this the other night, but couldn't pen sacs be made from TPE also? Like silicone, feels a lot like it and it's used in medical implants and can be moulded.
Just wondering, do they come small enough to fit a Peter Pan pen? In the next few weeks I need to get a sac for one and I'm going to replace the sacs in all my Duofolds, and Doric which have the old silicone (vinyl sacs).