I tried rubber washers, but didn't like the
way they stuck out, (too thick).
I used white Delrin washers on the light
colored part of my panel and clear plastic
washers where white didn't look right,
(black sub-panel).
I also waited until the paint was dry and
seasoned so the washers wouldn't stick when
put under pressure.
I also have an 'Affordable' panel and the
washers are all available at McMaster-Carr.
Tom