Whatever you use, it has to be a material that can stand years in total immersion of hydraulic oil. Oil is oil, plastic is made from oil. You have to do some research to be certain that the plastic you're using is going to last. If it disintegrates it'll probably damage your pumps.
There are grades of UHMW polyethylene you can use. I know for certain that any grade of aluminum will work. If all you have is a drill press, a piece of UHMW rod from McMaster Carr will do the job. If you have access to machinery, I'd just make it out of aluminum. Remember, one end of the piston/rod assembly comes off. If the piston is welded, the trunnion is screwed on. Possibly pinned. If you're going to disassemble, you might as well throw $5 worth of seals in them. And you don't have to make a split sleeve. Just a tube.. I would definitely put something the largest diameter I could find. The gland is made to have the top-out load distributed across it's face. If you slide a piece of thin-wall tube, your force will be as far away from its restrictionas possible. That's the gland's thread. It probably won't take long to fatigue that part.
