Another vote for the Rythmik Audio F15HP. Get 2. Build quality on these is second to none. I’m using one with my 3.6s as well some subwoofer towers.
I like the idea of the open baffle/ dipole except for the extra build work involved. Despite the theory, I’m not sure given the large wavelengths at sub frequencies OB/dipole subs would make that much difference in room modes. You could use minidsp /REW to flatten the response and equalize the subs (only) at the listening position and possibly end up with a better result.
Oh, You’ll have useable response to below 20 Hz with the F15HP.