John & all,  MDF has no, hmmm, ah, "resistance to stress", I guess I'd
call it.  Vacuum reservoirs tend to be under tension, and the MDF will,
over time, warp in response to that tension.  For example, you build a
shelving unit, and the shelves are MDF.  You will find that, if you put
any weight on that shelf, it will give and develop a "sag" within a few
months.

My solution to this has been to put a 2" x 2" stringer across the
underside of the shelf; so far, that seems to be working.  Reservoirs,
however, seldom have the space available to reinforce like that.

Plywood resists these forces much better, and would be my choice.
I would look for the multi-ply versions found in the higher grades of
plywood.  I think I would even look for the paper-faced plywood used
for billboards.  It will be more air-tight, and usually has more plies
than a comparable sized piece (7 plies instead of 5 plies).

David Dewey