I have a piston that I made from a 3" endcap cut into slices glued over a piece of 3" SHD 40 pipe. I glued the neoprene rubber seal to it with superglue. Superglue holds rubber to PVC GREAT!!! mcmaster.com
recommends superglue only when glueing rubber. I have test fired my gun to 50 PSI and the rubber seal never moved/came unglued. Use tons of superglue and forget the washers/bolts. It works great. You can use my valve design if you want. I only used a drill, powersander and a 10" tabletop saw. That's all the tools you need to built my valve. Hope you succeed with your new gun!