[Rimrock]

There are pretty tight tolerances concerning hydraulic seals and the grooves they are fitted in. The seal on a piston or rod works that way that oil under pressure are to come in behind the seal, often a 'o-ring, and push it towards the rod, or cylinder wall for a piston seal. It's not the pure friction of a seal towards a surface that does the main sealing alone here, so a gasket being 'tight' on to a rod doesn't mean it will seal under pressure (can seal to a certain degree if there are no pressure, or quite low pressure) (Not all manufacturers of RC hydraulics follows, or maybe are aware of, these principles, like Magom that preferes using quite 'beefy' soft o-rings and not letting any oil come in on the backside helping make a tighter seal, and it works quite okay)
Of course it is allowed to try make one owns gaskets, but as few/none make their own o-rings, there are little experience about what materials and how to make them. O-rings are mainly made of neoprene, a compund similar to natural rubber but more durable. Material for o-rings can be purchased on a roll, and one can cut to length and glue together. But this mainly used for applications where there are little/no pressure involved. With a rod seal made of a plat rubber seal, it will be difficult for any oil come in and press the gaskets radially on to the rod. Personally I would say it is a waist of time even to consider making hydraulic seals by hand, one have more than challenges enough with making grooves for standard seals, and other machined parts of a hydraulic cylinder, to tolerance. Don't misundersand, generally it is very good and important being curious all the time, being creative, learn new things, ask questions about things, how to improve things and so on.