What kind of vehicle are you putting the sub in. 240 watts might be a little much for one 10 w0 so be careful with it. I built a box for 2 10 W0s recently and had them at .75 cu ft and they sounded pretty nice. Since you got a little more power on it then I had you may want to drop down to .6 or so. Remember thats internal air space after the woofer is installed. Also if you have the room build the box with the most depth possible because that usually sounds better from my experience. Good luck with it.
Headunit: Eclipse CD8443
Front Stage: Boston Acoustics Z6
Rear Stage: Boston Acoustics Pro 6.5
Subwoofer: Dual JL 10W7 Sealed Enclosure
Mid/High Amplifier: JL Audio 450/4
Subwoofer Amplifier: JL Audio 1000/1