This is a picture of the piston that I used inside of my Mini LRPL-01, I looked and looked for parts to use for it but all that I could find that would fit inside a 3" chamber are these things that are called like tub hubs (maybe?), or shower closet hubs, anyways, I got two of them (at 5 bucks a pop, this piston cost a lot of $) and glued a piece of pipe inside them to hold them together and to give them extra strength. For the piston sealing face I used a temporary (sp?) end cap, you know, those things that are a black rubberly stuff? and they come with a free hose clamp too!!! Plus, they work great for the sealing surfaces.
Ok now the picture below is of the inside of the Mini LRPL-01's 3" chamber. You can see the 1.5" PVC elbow thats inside of it. This is what the piston pushes up against to seal off the barrel, it works great.