It's been a long time since I followed the car audio scene, but you can pretty much squeeze some subs in where ever you want, if you're willing to go through a little trouble. The way to make the most of whatever space you have is to use the floor/wall of the car as part of the enclosure. It might not be the best thing for a number of reasons, but it can make a huge difference in your sub options. It can take a bit of work though, since your budget isn't going to allow for you to pay someone else to build it for you. At the very simpliest, you can just use some mdf to make the mounting surface of the box and whatever sides you need, then liquid nail it to the sheetmetal. Using a bit of fibreglass to stiffen up the metal and the joints would be a good idea too.
You might also want to check into some subs that have very shallow mounting depths. I can't really speak to the quality of any current brands, but some models even have the magnets mounted on the front to decrease their depth requirements.
