Go buy some 3/4" PVC. Turn down the outside to press fit into the receiver(remove those junk plastic rings) and trim to length. Then, bore out the inside to allow a .002" fit on the cylinder you plan on using. Bear in mind you may need to dress up the ends of the pins n the cylinder, they can stick out .010 or more, depending on what *********** Charlie ate for breakfast.
I'll get some numbers on a piston tomorrow.