Thank you all for the suggestions. [Mcmaster-Carr is great!].
The valve rotates only when backwshing - so a few times each month. But
still, not static.
In speaking with RocketSeal, it appears a custom "vulcanized" gasket would
cost several hundreds of $$ just for the jig. [The radial sections between
the inner and outer O-ring complicates this fabrication.]
I could make a wooden mold with a router but I doubt I could pour a durable
gasket.
Since rotation is minimal, before I give up, I may try to fabricate a gasket
of appropriate thickness.
Again, I appreciate your ideas!

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