The rubber washers are only intended to compensate for an irregular mounting surface. If that is the case, I would sandwich a piece of Starboard between the trolling motor and the mounting surface and mount with bolts, lock washers, and nuts. I have not had great success with nylon lock nuts in some applications. Heavy vibrations tend to cause the nylon to deform and loosen.
2015 Seaark 1872 MV CC Etec-90
Helix 12 CHIRP SI, HB 1199, MinnKota Riptide ST80/i-pilot Link, Bob's Hydraulic
Bitter Gun Owner