I am a practical user, I am in no way a plastics expert but I use quite a lot of the two part plastics which are available in cured durometers from rubber band consistency to as hard as acrylic. For a small part like a seal you can make a pattern from modeling clay, wood or metal, use this to make a mold, then pour as many actual parts as you need. If you do not like the strength or size of the first result it is easy to try again with a different durometer material or a rebuilt pattern and new mold.
All this work is done cold. It helps if you have a small vacuum chamber to de-gas the freshly mixed materials and some sort of pressure pot (40 to 60 psi) for the final molding process so that the project solidifies under pressure which reduces any remaining bubbles in the mix.
I once had a positive shape for a mold made by a local shop. They scanned the shape of the original broken part into a CAD program, repaired the worn sections in CAD and then printed a plastic model of the rebuilt part. I then used this model to make my mold. Magic.