Probably coating the entire plywood covered wing, inside and out with
cladding epoxy would eliminate the changes brought on by humidity
variations.


You may be familiar with "Wood/Epoxy Saturation Technique"
That has been used in boat building for decades?

How about "WEST systems" epoxy?

Yep - that's where the "WEST" in WEST systems comes from.

The wood gets soaked all the way through, so the wood is just there for
support, and it seems to last forever.
I BOILED some samples for 24 hours with no degradation other than a bit of
color change. Add light fiberglass over that...