if you want to use wood washers you can make your own. They can be round or square
your choice. Keep it simple.
I agree with F.Ciccarello. The lite ply washers are simply to distribute the pressure of the nylon bolt head when tightened so they don't suck into the intercostal over time.
Rubber and metal washers are too heavy and overkill for the application IMO. In the big Patty thread I read much discussion on how to make the nylon bolts easier/faster to install but never anything about how often they were falling out.
Then again, what the heck do I know, my DA-50 doesn't arrive until tomorrow!
Good luck.