There is one URL at http://www.webjiin.net/rom/04_speakers_diy_4.htm
that you may want to have a lot. Unfortunately this is in Chinese. Yet you should be able to see the plans and the crossover there. It is using a Seas Millenium tweeter and the Skaaning driver.
The danish magazine, High Fidelity, also had a design featuring a 7" Skaaning driver and a Scan-Speak 9700 Tweeter, the XO, IIRC, was a 6 db/octave series filter at 2.4 kHz.
However, my entire collective of these magazines were lost (34 years worth of magazines) when I was buglared 6 months ago. Maybe somebody can help you in regards to details on this system.
Besides the whole point of the Flex Units is that you can order them according to your own specifications, the only thing that really restricts you is the laws of physics, as well as the size of your vallet.