Try a 2-017 O-Ring. Any decent hyd hose shop should be able to get you taken care of. May even be able to hit a local hardware store for them.
It should cost you about $0.20 or less for one.

Parker O-Ring size guide for 2-xxx sizes.
https://www.parker.com/literature/O-Ring%20Division%20Literature/2-xxx%20sizes.pdf