I have made boxes for the speakers for under the seats before in one of mine, ( I have also just lobbed bare speakers under the seats, the magnets hold them in place
). as for a sub woofer or a bass-ier ( I know that's not a real word) sound, just chuck an old house speaker in the boot
my mate used to smoke around in his MKII Escort with two huge Phillips house speakers in his boot, brilliant deep bass note .... he played some god awful music, but brilliant sound quality
.