This is a cool answer, but if the diffuser is behind the speakers most of the sound is already going to the listener without going to the diffuser first.
Diffuser panel design - overrated?
Often wondered about all the variations you see on diffuser panels. The interlaced cut squares. Some companies offer mathematically and scientifically designed panels that are supposed to perform extremely well. By Specifically dealing with all those individual frequencies with carefully measured elevations within the cut squares. How the heck do they do this given all the harmonics that are involved.
I mean it's just pressure amplitudes going across the room and instead of being reflected off of a flat surface they get scattered when they hit an irregular surface. And this provides improved clarity because it reduces the intensity of reverberations. Very similar to water being scattered when it hits underwater irregularities versus a flat wall.
So just by rearranging your bookshelf to create an uneven surface area, isn’t this achieving the same thing. It's not rocket science.