The fastest way(all parts local), is to build the PVC coupling piston.
$3for the coupling
$6 for the bolt, nut, spacer and washers.
$2 for the neoprene
$1 for oring
$12 plus a little for my time

BTW, I can make it without the seam in the middle, this was a prototype.
I can also do solid HDPE if you want, but it will be more expensive upfront. It's $20/ft on Mcmaster. One foot should yield 3 pieces.




