Martin J King has done a lot of work on open baffle systems. For the bass driver, it seems highish qts drivers work best. U frames push the qts higher, and Fs lower. H frames do the same but a bit more. Both U & H frame result in a loss of efficiency. Sensitivity guidance for the drivers a bass driver with 6-10 db more efficiency than a fullrange or mid/tweeter units. U frames ideally need to be rolled off electrically from 175hz, H frames from 150hz. Acoustically, those roll off points are higher due to both dipole roll off baffle and U/H frames boost in the upper bass/ lower mid region. Relying on driver acoustic fall-off alone is unlikely to work. This means one of three possible routes: second order passive (large value inductor & caps needed); analogue digital (lots of extra electrical gubbins to degrade SQ); implement a digital crossover with something like a miniDSP. I will be going with option 3, although I have the components for option 1.
For the full ranger, electrical high pass roll-off from around 500hz then should be sufficient. An Lpad may or may not be needed, depending on relative driver efficiencies.
The big 'must do' is to put at least a meter between the baffle and wall. Needless to say, this is very different from the semi-omni cube type approach. However, OBs are supposed not to excite room modes in the same way as box speakers, and those in adjacent rooms are far less likely to be disturbed by normal listening levels - in theory anyway.
Anyway, all of this stuff is a for dedicated post!