sierrax
+1y
1 Image Dynamics 10". I think that its a 1.2 cubic foot box (inside with subwoofer displacement), with a 3" diameter port, 18" long with an angled face (about 20 degrees). I did one of these boxes and it meter's at 140.8 db. Its the loudest single 10" that I have ever built. It took 4 boxes to get to that stage but that was the end result with the dimensions that I just gave. If you want I can get the exact specs for the box.
Another route is the slim pioneer subs. Two 10's or 12's of those pound good for the price. Sealed enclosure size is about .5-1 cubic feet.
And rember that your equipment is only as good as what you put it in. A cap on your sub amp, good wiring (proper gauge, ground), battery power, RCA Voltage output, enclosure quality and properly matched subs and amps will make a huge difference. A single battery system will only be able to put out 600-900 watts, battery depending. So even if your amp is 2000 watts, you don't have enough battery power to use it all up even if you have a crazy alternator.
Be careful if you do this. First measure the Ohms of each of the voice coils. And then find out what the amp can run at. If you run at too low then you might end up overloading the amp.