If you look at Graph 1 you will see a representation of a frequency graph for the given speaker dimensions, where the frequency will roll off below a specific frequency (50 Hz in this case) at a rate of 6dB/octave. If you look at the open-baffle graph I have included, you will see the sizes and corresponding frequencies for different sizes of speakers or baffles. This chart was found in Sound Practices, but this chart was found on the Bona's webpage (see link below).
The Bona's Audio Webpage
With the cabinet size, and looking at the graph just above, it is clear that the roll-off will be at approximately just below 50 Hz (I will include a more detailed description of how I calculated the baffle width for this speaker at the bottom of this article).
Now, for good low frequency response, and to make the general design easier, we will take a point of two octaves below the open-baffle roll-off as our lowest frequency. Two octaves below 50 Hz is 12.5 Hz and at this point and first-order X/O is instituted. The effect is shown in Graph 2 (ignoring for a moment the effects of the open-baffle).
If the responses of the two graphs are combined, the net result will be a flat response between 12.5 Hz and 50 Hz.
The response will be 12dB below the response of the main driver at the X/O point though if the drivers are of the same (combined) efficiency as the main full-range driver. The PHY-HP KM30 driver is 97dB/W, so this makes the choice of bass drivers difficult. The unit used needs to have a very low Fs, as the Fs is usually the lowest point I’d advise to use a driver to in an open-baffle system. The best of these I know of are the TC Sounds 8 Ohm, 15”, underhung voice-coil units. They have a Fs of 15 Hz, which makes them very nice for these purposes.
As I said before, a pair of TC Sounds 15” underhung coil drivers might be good in this application, but others like the drivers from TAD-Pioneer and JBL-Pro drivers can also be used.
Other driver to consider are units from Fane. The Fane Colossus XB series are very nicely built, but these, as in the case of the JBL-Pro drivers*, are only available in 15” and 18” drivers, and if the 18” drivers are used the cabinet must again be resized (* there is one JBL-Pro bass driver of 12" diameter, but it has a high Fs of over 50 Hz, which makes it less than suitable for this application).
I might have missed some very worthy candidates, but this is where individual builders themselves will have to decide for themselves.
Good luck and enjoy.