how do i measure cubic ft.?

my friend and i are planning to make me a fiberglass stealthbox for my IS250, and i wanted to have about 1.0-1.1 cubic ft. of space in the enclosure, how do i measure or produce 1.0 cubic ft. of space? i was told you could tell how much by using water and converting the volume of water into cubic ft., but the issue is, the water expanding/molding the wood ring. i was considering using sand? would appreciate anyones input.

Shipping peanuts. Or for a more accurate measurement, pellets out of a bean bag chair.

thanks craig! you always got the solution :P