Dax , it is soundgard board, this came in 4x9 sheets,he bought it at an auction for 11.00. Enough to build about 4 like that. I've seen the same stuff used under siding on houses(not great sheathing) ,just make sure you don't get the asphalt impregnated stuff! Nasty arras I bet! LOL
It is 1/2" thick,he cut it 12 inches wide and stacked it to get the height wanted. A board on top and bottom,some long bolts between boards to clamp(rebar with a plate welded to bottom and an allthread welded on top to tighten)
So I guess it is about 4' x 4' x 1' thick