i have a 94 cavalier. basically the same body\trunk\inside.
the back seat comes off with 2 bolts (underneath the front. i believe its like 11mm or 1\2in head- not sure)
take all that crap off, get rid of the seatbelts, take the plastic moulding thats around where the seat-backs are... remove the backs of the seats, and take up the carpet in the trunk. i would advise a good amount of sound deadening if you have it.
then get fiberglass, and make a mould of the metal where the back seat sat (all the way up onto the rear deck and seal that entire area ... construct a baffle of MDF that slants from the rear deck to the front part of where the seat was... and make some fiberglass (fleece) stretch to go round the edge of the baffle to your mould you made. seal it all round the edges with expandible foam, and 'glue' it down to the metal with the foam (stops some vibrations too)
its all theory, but i think id work with a little fancy fiberglass work... then put a nice huge slot port down the center. but your trunk will be like 15 cubes 0_o
prolly more. so i dunno how youd measure that. LOL //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif