Everything I found so far has been shaped like a D, but a round (or square) cross-section. I want the opposite (a circular ring but with a D cross-section), even in cord stock. oh well.
Meanwhile, the more I think about it, the more I worry that the RTV silicone will break down some day as the edge of it is immersed in diesel continuously whenever the tank is more than 2/3 full, and kept wet by splashing the rest of the time.
Hylomar and the Permatex version (available locally) seem to be the ONLY gasket sealants that are rated for gasoline and diesel exposure. I'm going to take the lock ring off again, and coat the sender flange, the exposed part of the O-ring and the cured RTV with the Hylomar. That should keep diesel off the RTV and everything dry. I hope.
I'm just tired of aftermarket parts being poor quality or defective right out of the box. But even if I were willing to bend over for Ford prices, not everything is available in Genuine Ford OEM parts for a 25-year-old truck.