medium density fiberboard is [u]not[/u] a good choice for speaker cab material.

As you've already pointed out, the stuff is heavy beyond belief. Worse, it is not as stiff as one would expect, making it a poor choice. To add insult to injury, the fibre content tends to absorb moisture, causing it to fall apart as the screws rip out of the joints, and the glue you put it together with pulls the top layer away.

Your best choice is a good grade of 3/4" or 1" birch plywood, with 2x2" or 2x4" bracing. The idea is to make the walls of the cab as stiff and rigid as possible. You should have a serious brace across the front panel between the speakers. A 2-by-4 on edge is minmum for a 2-15 cab. Use 2x2's along each seam, and across each panel in a couple of places. Use a good wood glue like Pro-bond or WoodTite. Epoxy is even better, if you can afford the cost. Use screws to hold everything together while the glue hardens, and then leave them there. Bass cabs in particular, experience extreme internal pressure variations, so it should be strong enough that you would be confident that it could survive a tumble down a long flight of stairs.