My first enclosure, do I have it right?

[sextiva]

Okay first off i have 2 Type R 15's 4ohm in the back of a 96 grand cherokee. My box design is a slot port, subs up port back.

The dimensions of my rear cargo area are 42.5(W)x19.5(height of backseat)x28(deep).

My box dimensions will be 42.5x19.5x26(externally). When calculating internal dimensions I subtracted 1.5 from the heigth and depth and 2.25 for the width when calculating the subwoofer chamber since there is technically 3 pieces of birch .75" thick. That equals 9.5 cubes internal chamber after sub displacement, and 1.2 cubic feet for my port(10.7 total). 4.3x18x26(deep) port area which should produce 28hz. Are my calculations correct and is 9.5ft³ too much for those subs? I have 1 alpine amp per sub that is rated just at RMS of the subwoofers(750).