If you want to have a go with a 12", then why not! I was considering the size of the enclosure!
What you need to consider is that the larger the sub, the louder it will be, and as you'll be that much closer, and with no obstructions, it won't need to be as loud as it would when in the boot. Best to match the driver to the amp you'll be powering it with.
I'd go with a 10" in a sealed enclosure, but do what you want and make sure you post some pics.