You could also go to the plumbing repair section, where they have faucet washers, and pick up some. They are fairly stiff and black, but not as soft as rubber. They'll conform around your components without cutting. You can trim them to the OD you prefer if you'de like, by using a razor on a cutting board (if they don't have the small OD ones).
I used soft vinyl washers that did the same thing, but the imprint is permanent on the washer. You can tell them from the hard plastics like delrin by scratching them with your thumbnail.
You could also use O-rings.