How to determine the volume of a fiberglass box?

Great advice,thanks for the tips.  Brian

The resin won't melt the foam?  I have never tried it so I am just curious.

7.481 gallons of water is equal to one cubic foot of space.  Estimating the cubic footage can be done with styrofoam peanuts before you resin the cloth. Move the speaker ring in or out or change the shape a little by braces or whatever until you get the desired amount.  If you make it a little bigger, you can always fill it with something to shrink it down to the desired size.  after its all glassed, I like to use the water method, this tells me exactly how big it is, and checks my work for leaks.  good luck, hope this helps.