tiltmaniac
Zen MBB Master
18-8 Stainless Steel with Neoprene Rubber Sealing Washer
for Number 10 Screw Size, 0.2" ID, 1.125" OD
After trialing these for a few months, I'm far happier with this solution for mounting seats than other things I've tried. This washer has a slightly domed stainless washer on top, and a rubber bottom. I use this one with M5 screws (it is just about perfect for those, assuming you like a tight fit).
McMaster Carr to the rescue: https://www.mcmaster.com/mv1495681275/#catalog/94709A613
