lol I had to pass geometry to get mine.
but it's pretty simple, multiply the width times the height times the length and you get the cubic measurement. since the trunk isn't a perfect box, just do it in sections. like the space over the hump would be one section, then the rest of the trunk would be the other section, and add them together.
Another way is to build your box as big as you can, coat it with fiberglass resin so it will not get damaged, fill it up with water, measure how much you poured in. All you need figure out how many square inches your bucket holds. I had to do this with a box that had many concave shapes and weird angles and it worked well.
FYI, good luck and best to fill it outside the car - also remember you'll have to get the water out somehow