Adding polyfil to a subwoofer box?

[CasualCatastrophe]

I remember reading somewhere that adding the right amount of polyfil to a subwoofer box can increase the "perceived" airspace by as much as 50%. That is to say if you had a box with 1 cubic foot of airspace then adding the correct amount of polyfil could make it act like a 1.5 cubic foot box. Is this true? Is there a way to figure out exactly the right amount of polyfil to add and how much that amount will increase the perceived box volume? I'm installing dual 12" subs in my brothers Jeep Wrangler and anyone who's seen the "trunk" in one of those knows that there is very little space back there. The subs require .88 cubic feet each, but a box made of 3/4" MDF with a volume of 1.76 cubic feet would be very tricky to make. It would be rather easy to make a box with about 1.25 cubic feet though so I need to know if I can add a certain amount of polyfil to increase the perceived volume to 1.76

Thanks a million to anyone who can answer this!