You could do a pair of DC 12's ported with that space. Hit up mylows10 if you go with DC. Or you could do those Icon 12's with a couple of PR's //content.invisioncic.com/y282845/emoticons/nerd.gif.c6fa51ddf7ff75f1c0371fbc648f70ae.gif
You will not be disappointed in the Icon. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif I would have voted for an SSD or Q.
or cut out some metal from rear seat opening, assuming your car is similar to my 2000 Altima. I've run 2 15's in 8 cubes and currently run a single 18 in 6cubes.