Look how GIK does it. By slotting out the side of the panels they take an effective surface area of 8sf and add another 5.5sf. That's not an insignificant addition to absorptive, non-reflective surfaces per panel. Use a standoff and you add another 8sf as you're exposing the back face of the panel to reflections off the wall as well. For maximum absorption performance you could skin panels in burlap without the use of an external frame but that has some pretty significant aesthetic drawbacks so we take the performance hit. Further, by constructing this way it's possible that some scattering is introduced in the higher frequencies. Not quite diffusion and the depths are so shallow that the scattering will only occur way up top, but still a potential ancillary benefit.
If you can improve a DIY product, which is something entirely within the control of the person building the product, then why not look at what professional-grade commercially available products are doing and emulate the things they've done which likely came with lots of R&D by people much smarter about product design than a DIY'er? Why settle for "good enough" when a little bit of work can yield something better? I'll never understand that mentality.
But I'm learned! 
