There is a "trick" to greatly reduce feedback issues on guitars with a non-flat or arched back. I found this out through trial and error and it definitely works. I first followed the video instructions on the TA website and placed it so that the rails were as flat as possible. This is wrong. My guitar fed back like crazy, and yes, I did the "seek & destroy method" notch filter thing and set the master gain first. I was about to give up but then I found that by slowly pulling the unit slightly away from the back of the guitar the feedback got worse and conversely, if I pushed it towards the back of the guitar the feedback lessened dramatically. What I ended up doing was putting padding underneath the right end of the unit to raise it up enough so that the round transducer was flat against the back (it was not flat before but was only making partial contact). Problem solved and the unit was still attached securely enough even with only one of the rails uncovered (protective film removed).
I don't have feedback issues, but I do find best performance is with the transducer's rubber grommet (TWA calls it the 'cone') flush against the back of the guitar. I was trying to get it flush, but I was putting padding in the center back of the TWA, and the rails were too much out of contact. I'll try putting a rubber washer under the right end of the unit as described in your post and give it a shot. By the way, I find that when the grommet is flush, the resulting volume increase is very significant. A 'night and day' type of difference.