I have another idea to go beyond the idea of a flat "board" of plastic to swap in.

The chair leg itself is pulled into the frame of the seat as the bolts are tightened. It seems to me that this creates forces directly in the center of the brace where the bolt heads rest and the edges of the brace where they contact. Not unlike the span of a bridge.

This line of thinking led me to a structure that is often used to support spans at length. The Arch. So my replacement brace would be an arch. The problems I begin to see here are the span length, filament / printing direction, and any complex geometries that may not be easy to print as a single piece.

This also led me to on consider one of the architectural elements of the arch. The Keystone. The keystone effectively being a wedge at the apex of the arch's span. Using my own wedge or keystone would allow the design to naturally be broken up into pieces. I could use a central wedge and print separate "arms" for the span.

This distributes the forces differently. The span arms are pushing on the wedge of the keystone and frame of the chair.

My thinking here is that as the bolts tighten, the wedge moves closer to the chair leg. But this is met with the opposing force of the span "arms". It also allows me to print the design in three pieces instead of a single monolithic piece. This should accelerate the design process as any problems will not necessarily need an entire reprint. It should also allow easier scaling and access to more printers as the individual parts will be smaller.