Comment by pjc50

Dredged from memory is the "phenolic" circuit board, popular before about 1990.

https://picamfg.com/pcb-base-materials/ "FR-2: Phenolic Resin with Paper Reinforcement" / https://epra.eu/en/sustainability/bio-sourced-and-bio-based-... - you could make an entirely natural-derived paper+resin circuit board, with high dimensional stability, and validated by real use.

The only downside is it's not inherently fire resistant.