Imho is clear
99 lt and no us-box as you can use sail up to 8,5 according to specs, and so you need a long fin, imho 40 that is the upper end of fin range. Us-box 40 cm is not recommended, too leverage I've heard.
So tuttle or powerbox. No powerbox for brand choice, I don't understand the compatibility between single rear straps setup and the two screws of tuttle system.
Anyway imho a great idea and a great board, I'd like to test one.
P.s.: at 1'00'' of the video can clearly be seen an us box on the bottom, did they change idea before production?