manipulating enclosure volume

i am going to build a fiberglass enclosure to fit part of my trunk, but how do i build it to specifiations? im building it to conserve space, not just to look cool. should i build the box trying to make it a little big, then just measure it with packing peanuts and displace the volume with mdf? i really have no idea what to do here. also, what is the benefit of using kevlar? (assuming money is no object) do i need a different kind of resin? can i use less kevlar vs fiberglass? thanks for the help

 that is correct build a little bigger...calculate volume with packing peanuts ..i use popcorn kernals a little more accurate add mdf in form of bracing (to kill 2 birds with 1 stone) until correct internal volume is achieved...as far as kevlar never worked with it ....

Thanks, i read to use bean bag beans too, we'll see what is readily available when the time comes. i might be doing it on sunday so it will be an adventure. thanks for the help