If it were me, I'd go completely overboard and saw 20mm off some 6mm thick 20mm wide mild steel bar (I've got some lying around which was bracing a door, preventing burglars from bending the wooden door away from the deadlocks, which happened to my neighbours. The door has since been replaced with a big UPVC triple locking version), drill holes into it and use those instead of washers... which wouldn't at all be appropriate to the cargo weight you mentioned. Plus mild steel would rust, so I'd have to buy some stainless steel bar from ebay...The holey design is one reason I went for the D rings nearer the edge , the hooks trying to grab hold of the ply base through the floor holes is fraught with issues, not least slipping from lack of a good anchor.
Washers I doubt will bend they are approx. 1.5 - 2mm thick.
Todays cargo load.
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