well you could make the bottom pretty easy, with just a piece of plywood cut to your arc. Then place 2x4s with the 4" side flush with the arc of the plywood on both sides, maybe 1 foot apart. then measure, say 2' from the bottom of the 2x4s on the inner arc, and 4' from the bottom of the 2x4s on the outer arc. take a meter stick and connect these two lines, cut along them using a wormdrive saw. this will make the 2x4s have the angle of the c-box, so you dont just slide off. then just patch up the sides with plywood if you like, and surface it with puck board. It's not too hard.