Quickshifter Refurb Advice

[Slim Chaney]

I picked up a quickshifter off ebay, complete with cage. Theres a rubber gasket/seal/washer type thing on it thats broken, as in about a cm missing, which is causing some gear lever wobble. The main fulcrum of the lever sits in a spherical recess, theres then a nylon top piece that gets dropped on top then theres that washer sitting on top, compressed when the top of the cage is closed.
Total longshòt but has anyone come across this part when searching for other things? Im wondering if I could replace it with 2x2mm o-rings stacked on to of each other but I rather try to get it stock.
I'll post some pictures when I get it back out to have a fiddle and a play to give a better idea of what Im asking.

I "could" use the washer from the cage in my car currently but I havent removed it to check condition. Ideally Id rather have 2 complete cages if possible that I can swap out.

Thanks.