Summary

Syringe type plunger and cylinder.

Instructions

I had hoped this would be a hydraulic piston platform which I could adapt for use for simple linear mechanical energy transference, which would have all of the adaptability of 3D printed parts. However, based on the effort required vs. performance, I think I'll pursue a different method. The design is as simple as a plunger shaft with 3/8" inner diameter by 1/8" cross sectional diameter nitrile O-rings as seals to the outer cylinder.

The first problem I encountered was the outer cylinder not being water tight. This was fixed with a coating of abs dissolved in acetone. (that stuff is great by the way, I highly recommend painting a layer on your print bed to insure print adhesion, as always be careful when handling acetone)
Similarly the fluid would leak around the O-rings on the plunger side, and this was fixed by dipping the ends of the plungers into the abs/acetone allowing it to dry and sanding the deposited abs off of the O-rings.
Finally, even after those modifications the fluid would still leak around the outer edge of the orings which brings me to an idea that I think would be better.

I believe the design would be improved by a nearly optically smooth inner surface of the cylinder as I think it is the ridges that are causing the remaining leaking, which makes me think that for my next attempt, I'd use actual plastic syringes, and just print replacement plungers and a holder for the syringe cylinder. For larger pistons, I would expect PVC tube to be smooth enough to be used as the outer cylinder.

Hydraulic Piston (simple Syringe type)