I think the ultimate would be to fine a suitable sized rubber cork to use in place of the pvc pipe cap.
I made this 'temporary' tampion awhile back and never got around to making its replacement:
The rubber 'cork' is a Neoprene stopper for a CAT track roller shaft. The shaft has a hole bored down the center from one end, pressing in the stopper turns this hollow cavity into an oil reservoir. The oil is pumped in through a hole in the center of the stopper, which is sealed with a ¼" diameter plastic plug.
The stopper I used has a max O.D. about 1/16" larger than the bore. It will get nice and tight about 3/16" from the big end. It can't go in too far, it would have to be pressed (or hammered) in to go flush.
The knob is a wooden drawer pull from Lowe's. I had to use a longer screw, which goes through the existing hole in the rubber stopper.