Martin J. King has some mathcad worksheets you can download from his website
www.quarter-wave.com. The best way to design is by trial and error, his worksheets do a good job of predicting the freq response. In this case, the line is 70" long, and has a taper that is 2.5x the sd of the driver at the closed end and .25 at the open end. At this point, I just fill with .25lb/ft^3 fiberfill and then play around with densities a little higher and lower until it sounds right. TLs flatten out impedance spikes associated with other forms of enclosures (which makes an easier load on the amp), and because of the open ended design, the enclosure should theoretically be non-resonant. In my case, it won't be because I am tapering so severely. This was necessary as the Low qts driver limited the bass output on a more traditional design. The shorter length and strong taper got me an F10 at the same freq as a low qtc sealed enclosure but with an additional 4 dB of output and reduced excursion.
The subs are going to be Lambda PC12s. (two identical enclosures)