Thanks Shawn. I still question the portion of the instructions below. It looks like the dimensions of the inner tube disks are switched for the plunger portion and the bottom plate portion. Other than that I think I understand it.

Cut a disc of inner-tube about 1/8" smaller than the ID of the pipe. Sandwich it between the top plates, with the smaller plate against the 1/2" pipe. Bolt the stack to the threaded hole with a 1/4" x 1" stainless bolt.

Cut a disc of inner-tube about 1/16" larger than the ID of the pipe. Use a small stainless machine screw, fender washer, and locknut to bolt the bottom flange to the bottom plate.