The box i have for 2 12" subs is .85 cu ft per side, if i flip the subs upside down, would that cover the extra (i need 1.25 or 1.0 cubic feet)airspace I need?

 

You could probably get away with the smaller box just by adding poly-fill stuffing inside the box. Get it at walmart in the sewing section. I think your supposed to add one pound for every cubic foot of airspace. It tricks the sub into thinking the box is bigger by slowing down the sound waves or something like that, but it really does work.