I dont want to promote this idea as mine or sell it commercially because i think the company RGP Diffusors has the patent for this stuff...
I just calculated my own 1-dimensional QRD Diffractal for my needs

i think (i dont know and didnt measure it) they are diffusive from 500Hz to 16kHz
btw: these nice milled mdf boards with the N7 matrix in it are made by the carpenter and i paid around 250 € for 50 meters. this is enough for 6 diffusors...
not more to say... i dont have very detailed pictures because i wasn´t planning to do open a new thread about it but now i do...
i hope you guys like it and maybe its helpful... if u need some instructions for your DIY diffractal just ask

Andi