if this is for SPL and you're worried about flexing, do 1 piece of MDF, 2 inches of concrete, then 1 inch of mdf on the outside, like sandwich some concrete, you can get 48x48 pieces of lead in there too, you can buy those at a roofing supply store.
what is this project for?
the ultimate wooferbox is made of 9 inch thick lead so they say.. but for cars, concrete with lead sheets is as good as you can get