For the new seal, I am using 1/16" Ultra Strength Neoprene from McMaster, which I think will last much longer than the old seal, which was simple Natural Rubber Latex of the same thickness from the hardware store. The epoxy is still setting, but I hope it works well and for much longer! Because I would also like to make a more aesthetically pleasing gun that is not serviceable, and don't want the piston wearing out because then it'd be useless.
I also received the schrader cores I ordered: 9914B short cores from an aviation supply. They're rated to 4000psi and fit in any old schrader valve body. I hope this lasts long too, because I got sick of replacing valves whose seals were destroyed from the pressure.
Thanks! Really does mean a lot. I will try that sometime soon, sounds like it'd make for a really cool effect.