I just used generic 1/4"washers from O'Reillys & they worked fine.
Also used the Moroso blue rubber gaskets with steel in the middle.
$38, but they don't leak. You can see in these 2 pics just before I painted them, the round, hole bosses on the sides. The washers fit neatly on them & the corner bosses weren't a problem, even tho they're not round.