Alot of work goes into proberly designing a bandpass box, and if you dont take the time it will sould like those ones you can buy at walmart and best buy
The *best* bandpass enclosure is the Decware Wicked one. It can be made without any designing and can be tuned on the fly. However it's a little small for an Uberwoofer. $10 will get you the plans, and you can make them larger to accomidate the 15" sub.
Be warned, you'll need a whole sheet of MDF just for one box. I ran out making two of them for 10s.