It you are going to mount an 8" woofer I would suggest aiming it into the cabin. 8", though massive compared to say home audio subs are not going to give you a consistently audible sound while driving (or at least not when you're on the freeway with your windows down). I actually think that you've come up with a viable solution under the glove compartment. Build a sealed box out of fiberglass so that it contours to the firewall and any obstructions. You can find a great fiberglass runthrough at
http://www.sounddomain.com/memberpage/493978. This isn't a tutorial persay but it will give an idea of how to piece together fiberglass. You can email him as well by PMing him through sounddomain.com. He returns emails and is very helpful.
I haven't looked for your box on the web because I doubt you'll find it. good luck though