Find an industrial supplier. Looked at Summit's pricing and their pricing is almost criminal.

P/N: 600-0101-3/4 or NAS1523-12 should be able to find for about $2.50ea or less. They are the seals designed for bulkhead applications.

The metal cup and o-ring shown above already made by Parker as the 800 series Lock-O-Seal. Made for applications that allow trying/changing o-ring compounds.

Check hoseandfittings.com for the 600 series (thats where I got $2 price from) or aerospace parts dist for the NAS series.