I've heard good things about Valchromat, but I believe in the video they said it was quite expensive though, even vs higher grades of MDF I'm not sure about US pricing though.
The MUM-8 is a cool speaker, & I've been following PDP since the release of their 2nd video a year ago, initially showing off the test cabinet with the Bliesma drivers & Dayton woofer.
I do agree that they could use a bit more bracing though, along with some NoRez and polyfill/fiberglass.
I'm actually working on a personal project using the Bliesma silk mids and tweeters, inspired by the MUM-8 of course but it'll be smaller, paired with our M165 NQ in a sealed box rather than a Purifi woofer.
I'm just waiting on the test cabinet to get finished so I can start design the network and see if I need to make any changes to the cabinet.