If your gonna keep the 18s you may go 1 18 port up or trade them straight across for 2 15s if you dont want to sell. Ive seen some builds on other sites where guys go all out sealing that back window off before starting the back wall but i cant find the links right now. You have some options but i think you gotta decide what you want out of your system first. You might even find more efficiency out of 12s and more power in that air space either way im tuned in to see what you settle on!
somebody send db-r a dd-1 and write it off on taxes as a business expense so he can make a vid showing that its user error and this thread can be killed. lol
on a side note ive entered to win one from twistedchild so i can add it to the toolkit.
12 smd 12s in a flat wall or 18 smd 12s in a clam, steel framed glassed front smd style ,rockford bd1500 per coil on 16 volts tuned mid to high 30s on the wall or 1/3 cone area on the clam...60s or better on music haters go home