I'd FG the bish, those enclosures are complicated, and requier either a lot of FG'in or a lot of cutting...you kinda gotta piece it together...have fun cuz it's going to be a pain in the ***!
why not just add one more piece of MDF and make a normal box?
And if the back of the cab is odd shaped (could be the reason you don't want to just use a flat back board) then it's gunna be a ***** trying to calculate the volume.