Well the diff between a BP4 and a BP6 is similar from sealed to vented - free dBs for the vented version. You could try that 'BP4' box and then, if you want, try expanding it out the back and converting it to vented.

 

The problem with extending the box is that te volumes are greatly different between thr 4th and 6th. It would be way more work than building a new box. I have one more avenune I'm explorng then building. I won't be making the trek on the 26th but i will be attending deadlysones show on the 17th with this monster.

 

You can make either work with the same 'front' volume.

Again - if you would forget trying to tune it to a bandpass this would all be much easier, but to each their own.