Many types of plastic foam eventually breaks down, and once the process becomes obvious it seems to accelerate. I made it through this spring using my old Shure A81WS which are on their way out, careful not to rub or squash them too much, but I'll need to replace them this fall. I've decided to stick with the Shure's instead of moving to the Rycote's or Movo's partly because my application is rather non-standard and the foam Shure's are more adaptable and a proven known-good solution for me and I figure a decade of use before needing to replace them is reasonable.
I'm now wondering if storing them sealed in bags with some oxygen absorbent packs in there may extend their useful life. Not desiccant packs, but the oxygen absorbent ones, since many foams are susceptible to damage from long-term exposure to air, or even short-term exposure to ozone.
If anyone has or finds an especially attractive source for Shure A81WS please let me know as I'll need to pickup four of them.
I've considered going back to the DIY baby ball gags based around plastic mesh embroidery balls intended for embroidered Christmas ornaments, which I was playing around with a decade back and have great potential as they form an excellent transparent support structure trapping an optimal-volume still-air space, but the two problematic aspects are sourcing really well designed cover material and engineering a good pragmatic mounting arrangement. Easier to stick with and make minor modifications to the Shures. Would love the Rycote BBGs, but as I'd need to modify them as well, so they aren't a reasonable option for me.