Or just redo the baffle to fit 8 12s facing front.I have an idea.
Try working with some higher quality stuff, or more power, or something...
Nothing against AudioPipe or Hifonics, both make some good stuff.
But my opinion, if you have 21 cubes
Would be to rebaffle the front, and size the box down just a tad
And do four 15's on more power
4 12s facing front and 4 12s facing back with a port forward will not be good at all and tuning will be out of whack as well as the rear facing subs unloading in the port causing them to have over excursion and they can blow.