Hi John and other MMD folks,  I most certainly would NOT use MDF for
a bellows board or anything else that is a structural part of a piano
or other instrument.  But I also wouldn't use ordinary plywood from the
"big box" store.

At the very least, I would get poplar plywood with no voids.  Better
yet is Baltic or Finnish birch plywood.  Some hobby stores sell what
they call marine plywood, which is very strong, flat and dense.  You
can even get it 1/64" thick, which has three plies.  How they cut
a ply of wood 0.005" thick is a mystery to me.

Regards,
Craig Smith
Upstate New York