Maybe you should take a look at whats available i DIY car audio. There are many shallow depth woofers available for that. I cant recommend any though, as i simply dont know whats good and whats not.
An other option is 10" woofers. I believe you can fit those in 6,6" enclosures. If you can can build wide and tall you may be able to do a ported cabinet. If you can get another few inches to play with, you should be able to fit ordinary 12" woofers.
How much SPL are you looking for?