I know that maybe an eye-glass case is a bit of a stretch for a muzzle-loading accessory, but it has sorta become essential for myself at least. I discovered this on Amazon a while back and have been using it for a few weeks to keep my reading/shooting glasses relatively safe and clean in the possibles bag. It is apparently hand-made, out of soft leather and isn't just TOO expensive at $23.76. No plastic parts I can find and nothing that could break that couldn't be fixed in the field. Not too many hand-made items for sale in the world these days so I like to support the people who still do this kind of work:
https://www.amazon.com/Pablo-Leathe...0e4-4414-8454-48638ccc6f8f&pd_rd_i=B076GLBYYH
https://www.amazon.com/Pablo-Leathe...0e4-4414-8454-48638ccc6f8f&pd_rd_i=B076GLBYYH