I just picked up a locking gas gap from Jim's Auto Parts for the under the Flip Top Fuel Cap.
Works really good, Looks good, Fits like a glove, turn the key a 1/4 turn and it pops up.
Push down to lock it. Made of black plastic.
It has two solid tabs and two retractable tabs when you turn the key for the mechanism.
Not Original, But Affordable, and it does the Trick.