I can find an o-ring to go under the sender plate. I could even cut a gasket from a piece of that old inner tube.
Then I need some sort of thick solid washer to go on top of it. A rubber o-ring or washer on top of the plate, and under that jar lid looking lock ring, will not hold the sending unit in place. It will deflect.
So I'll probably end up making something for that, myself. Maybe 2 or 3 rings made from an old plastic cutting board or something. Too bad I don't have a couple plastic pipes that are the right size, to cut rings out of.
I think I'll look into that!
I'll try putting it mostly together in the next day or two, (Raining here.) so I can take a pic and show what I mean by the gap. Well, if my camera will even take a pic that is good enough to show the problem. Man, I need a better camera.
I'd still like a recommendation from someone for some kind of sealant that will stand up to diesel fuel.