There is no such thing.

Piper will be happy to sell you a locking cap for around $700+ (if
they have it in stock), but that is the only option. No one else has
an STC or PMA on that part.

I wonder what kind of testing is necessary to get an STC for a gas cap?

What a profit opportunity for someone! Heck, charge "only" $200 per cap,
pocket $180, and sell a few hundred of 'em...
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"