to get the exact cc you have to fill the enclosure up with water and pour that into measuring jugs to measure it 
LOL, somebody will do that!!! if you must, then put a black bin bag in first to stop the water ruining your hard work, or just work it out mathematically.
its fibreglass adam, water does nothing to it

just have to let it dry in the sun for a bit
i have never not used the exact cc when i make sub boxes, and they have always sounded great, but i dont really know how much difference it will make.