Here is an idea:
Just move the amps under your seats and use the space behind your seats for box volume, or stack the amps behind the passenger seat and use space behind driver seat for box volume. You can flush mount the woofer and cover in grille cloth to hide it from curious onlookers.
Don't worry about how difficult this looks on the outside, a box is a box. Building it won't be too hard (remember geometry class or were you daydreaming about the senior class prom queen?). Its covering your finished product that will try your patience.
Mal