In my opinion as a Cabinetmaker (Carpenters are those who frame and such) that I would suggest fiberglass as the best in air tight, strength, and weight. Regardless of grade of plywood, any moisture or humidity can cause warpage and/or shrinkage (cracking glue joints and other things). MDF is very strong, and pretty airtight when the right glue is used (I used gorilla glue, worked perfectly) but... it's weight is it's weakness. Fiberglass is when used correctly very strong, airtight and quite a bit lighter then MDF and yes, even plywood. I have never worked with fiberglass myself but have seen a couple of my friends work with it for multiple things.

That's my $.02
Cheers

:wood::UDONT::lame::thumbsdow

Carbon Fiber Nomex Honeycomb Core enclosure is by far the lightest material I have ever made an enclosure out of.

I made a 2 cube sealed box for a single 12 before that was less that 2lbs and was 1.5" thick(two .75" layers). before I put a .5" Birch ply panel on the front to have something to bolt the sub too.

Just google it you will find its not cheap but that box was approx $500 to build but customer was very happy with how light it turned out.

For a budget lightweight box just go with 11-13 layer birch ply its probably your best bet.

x2:):):):):)

I stand by 3/4" styrofoam laminated with fiberglass on both sides.