I am planning on building an subwoofer box for my wifes car but I want to conceal it. It is going in a Saturn Vue under the rear floor by the rear tire. I want to keep the spare tire and build around it. (there is plenty of room) I want it to go about a quarter of the way around it. What I was planning on doing is making a mold out of Pink Rigid Foam Insulation and then fiberglassing around it. But my question is how do I find out the the cubic ft of the inclousere with it being odd shaped?