Nick, running a sub free air like that is really bad for it. You said, "get a box" unless you get a box for a custom builder you'll be wasting your time. Prefab boxes are absolutely terrible, they're not the right size, who knows what the ports tuned to, untreated MDF, etc
I concur with Sippi. Alpine Type R's are good subs. I had one on my boat a few years back. You can build the box yourself. Check out my thread on box building, or you can tell me how much space you have and I can give you a cutlist, slot port or flared port. In the right box your (1) 15 will slam. I think an Alpine type R needs around 3.5-4^ft after displacement.