I don't know where I want to peak at. My current box is 4^ft net @ 34hz-ish and I like the way it sounds, or at least I'm used to it now. I don't know much about bandpass boxes but I know you can get more out of your woofers by using 4ths or 6ths instead of standard slot port boxes. I'm fairly new to car audio but I like to build things and I really enjoyed building my current box that they're in now. I just want to see if I can get more out of them. They hit a 140.5 at the moment.Just so you know you need two separate tunings for 6th order. The inner is your low side with very little port area the outer is high side with a ton of port area. Is the 33-36Hz where you want to peak?
x2i can assure you you do not have the money and time to get this right. If you're dead set on building exotic boxes for fun, experiment with a smaller one for a single sub. Understand what it is you're doing and how it works so that you can plan to be able to modify as needed once it's built and sounds worse than your current box.
Considering that picture doesn't come with dimensions it's safe to assume it just looks impressive and nothing is to scale and it's completely un-tested. Bandpass designs aren't free output. They're output at the expense of bandwidth... Or if you actually want a wide bandwidth you basically wind up with a worse-sounding sealed box.