have this same challenge, someone just told me the other day I could possibly grind the area of the piston where it is compacting the plug's electrodes together. Don't want to do that really though.
was just looking for washers last night, then today I saw this ngk pdf:
after looking at it, it seems for my application (stock newport cylinder with a Treats Hi Comp head) I would want to use a NGK BP6SS. the first S being the thread reach which S indicates 9.5mm or 3/8ths.