To people still following this topic. Yesterday I used a PET bottle and cut out a suitable seal for the GL45 cap. I had to sand down the small lip inside the cap first to make the inside flat, which was quite tedious, but about an hour with a sharp scalpel did the trick. A Dremel drill might do it in 10 minutes, but I didn't have one handy.
In any case I pressed my DIY PET seal into the cap, and then I put a full strength MMS solution into my bottle and closed it for 24 hours.
Today I looked at the condition of the seal, and it doesn't have a single scratch on it. No sign of any oxidization. Not even a blemish or stain.
So I'm going to call PET a go for this purpose. A longer test might still be good to do, but compared PP which bleached easily in 24 hours, this is night and day. Very happy with this so far.