These shim plates will increase the plywood frame strength as the lips prevent the plywood to split. Place the shim lips towards the bolt and the nut on the even side.
The very solid frame you'll get if you glue all parts (but Z rods top plate allowing the service) with normal PVA glue.
Sanding the black paint out from the glue joints wouldn't harm :-)
Before the glue is cured check that frame is square.
The original bolts & nuts keep the frame nicely in together during glue curing so no extra clamps are needed.
The author marked this model as their own original creation. Imported from Thingiverse.