Redesign of box for my ICON - help needed

[riored4v 8]

I built a sealed box for my 12" ICON to proper size for my mustang, but I've recently sold that car and picked up a small hatchback.  I want to stick with the sealed enclosure design for this car to minimize the amount of space.

 

Whats the smallest I could go enclosure-wise for the ICON?  Would it be possible to go smaller than the .9-1.25cu ft size thats recommended if the difference way made up with polyfill?

 

Ideally i want as small as box as possible but I still want the sub to sound great.