Here's a pic from my file; not the best I'm afraid; will see if I can find some time after the holiday to take some from one of the collection!
http://freepdfhosting.com/b339a1a905.pdf
PTFE is not sturdy enough but PET may be fine and, I think, readily available in sheets!
The discs do not last forever though but longevity can be vastly improved by smoothing the edges/corners of the cyl slot!
It would be very easy (although I have never tried it!) to cut a profile matching the piston slot and fixing with a dab or few of supoerglue or appropriate Loctite!
Can't recall who it was who recommended re-profiling the miushroom head since it was quite a while ago!
Anyway BrumBow fans, happy airgunning!
atvb
David