Some great suggestions!
The rubber pads that I received do not have any adhesion on them and I guess that's the way they all come these days. I have some of that spray-on 3M adhesion the only thing is I'm concerned about being able to adjust the pad before it sets up. I'll do a test on something with it.
The silicone sounds like it would work pretty well. Probably have to tape the pad on the tank while it cures. I'll do a test on something with it too and see how long it takes to set up. I think ten minutes would be to long. Maybe it depends on the brand and type. Weedie, is that the way you did yours?
The RTV application is something we were using even way back in the day, it had a double use, for an instant gasket material, or for making things stick without ruining either surface. RTV will not harm the paint it sticks to.
Ok here more recently, I bought two little tubes off ebay, just to be sure it was fresh, sealed in its hang card. AutoZone probably sells it the same way, whichever.
THE HOW TO DO IT:
What I was careful about, clean both surfaces,no dust or fingerprints. Use alcohol on t shirt cotton, good to go.
I spread the RTV on thin with an artist paint brush, cover it right to the edge. Spread it on 100% of the surface of the pad, dont goop it on the pad in big blobs, it will ooze past the edges. It "cures" right away. I don't think RTV ever really dries out. Good idea to do your own tests, get familiar with the stuff.
Obviously, you have to know exactly where to set the pad. I taped it on with painter's tape, used two horizontal pieces of tape to reference the pad position, and then its ready for the RTV.
I made note of just exactly WHERE the end of my knee hit the side of the tank, and this gave me the fore/aft position for the pad. The tank on my Thruxton is no peanut, so it took some fooling around to figure out where they looked the best, Try more than than one position.
The RTV should be the automotive type, not the silicone stuff in the caulking tube.
Be patient, have fun. and let us know how it went. Pics are great if you got em.