Great @clio09. The Madisound OB frames are real nice, made of Baltic Birch. Does Madisound sell the LX521 OB/Dipole woofer frame alone? I installed my OB/Dipole woofers in the same style frame (Danny refers to it as a "W", others an "M", and Madisound a "V". All the same thing ;-), not the more common H-frame.
One thing to consider is adding a second layer of BB or MDF to each side panel, to stiffen them up and lessen their resonance. And a sidewall-to-sidewall brace can be installed on the "open" (typically front) side of the frame, again to add cabinet stiffness and therefore reduced resonance. Another tweak is installing a layer of ASC WallDamp between doubled-up side walls, again to reduce resonance.
Brian Ding installs a 6dB/octave "shelving" circuit into his Rythmik A370 plate amp when used in an OB sub, to counteract the dipole cancellation inherent in OB woofers. That’s an easy filter to construct, just a couple of parts.