I did try 0.5mm ptfe (I think) once in a 25mm piston 77 but found that the spring was too tight a fit on the ptfe ... it would just fit the spring with a bit of a push but I felt it would get too tight with a compressed spring. So I reverted to a 'beer can' or maybe it was a coke can, liner. I have never had one fail me yet.
Cheers, Phil