if it's just sheet rock i would just cut several box fans into the wall. cut a flap out of that pink styrofoam board large enough to cover each fan and adhere it in such a manner that the fans lift them when on and they fall shut when the fans go off. you following me on this.
it sounds like you have plenty of room to do a little duct work from the a/c. if you can thats your best bet. if i were you i'd move ALOT of air through there. if your gonna use co2 then the flaps over the fans should keep it from leaking out all at once and if you make the flaps right they should seal up pretty tight when the fans are off
no co2 set up yet but im looking in to the co2 units that burn propane. + no cash left over spent it all on the the lights. im shooting for a a never ending supply. 40 plant rotation 2gal pots. clone for ice,bubble gum,haze/19skunk.and wh. widow. im look for a varity i get burnt out if i smoke the same kind every day. i will post pics. tomorrow....