In my opinion, that would be a massive waste of time unless you were just devoted to taking on a project. There might be a sub on the market that is a self contained unit (JBL, Infinity, etc) that MIGHT fit under there, but you could be running into problems with seat travel.
If you look into fiberglass formed enclosures, you can fit a 8, 10 or 12 inch sub into the corner of your trunk without loosing any real usable trunk space. You'll need to pay close attention to the sub you are going to use and ensure the sub will work in the airspace you plan to provide. It is a custom job, so if you're not able to take the task on yourself, you'll have to find a stereo shop or someone you trust (with experience) to do it.
I have a Skar 10 sitting on the floor of my cave just waiting on me to catch up on things and build a box in the trunk of my '94 vert. It's gonna happen. I promise....