I need a 109 seat.

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Hey hey, looks like you're still in the business of punching out your own seats!
I wonder if inner tube (for tyres) would work.

Hell, you could probably fashion an LP seat out of "shoe goo" but I would not count on it working long or well. I am looking at a few old BCD washer seals as potential material for LP seats for Conshelf 2nds. My punch is the right diameter but the material I use for SP seats is a bit too thin for the single adjustment Conshelfs (fixed orifice).