Dear John, My two cents...  Personally I would stay away from MDF for
building a reservoir.  My concerns would be the glued joints between
the pneumatic cloth and the edges of the MDF panel.  MDF would be prone
to flake away a bit, like cardboard, and transform to dust, making
those airtight joints unreliable or difficult to make properly.  The
edges of the panels would be a bit too fleecy to my taste.  Using HDF
instead would be better but not 100% reliable either.

Better to use the best grade of plywood, what we call Russian plywood
or Balkan's Birch plywood would make for a very high quality job.

Gilles Chouinard, organbuilder
Laval, Quebec, Canada