If you can’t find what you’re looking for, what about maybe taking a stack of the large washers with an oversized bolt hole and notching them in a star pattern on a band saw, then spot welding flat round speed nuts [A.K.A. Tinnermans nut] with the needed thread size centered in the oversized bolt hole.
The flat round speed nuts I’m referring to are at the bottom of this page.
Flat Round Speed Nuts They also make what are called flat anchor nuts and flat weld nuts, but they aren’t exactly what I would call flat, so don't know if they would work.
Flat Anchor Nuts Flat Weld Nuts 
