reflection phase gratings are relatively easy to construct, just ungodly time consuming - especially the 2-dimensional variety. although for the bandwidths most are constrained to in home residential rooms (1/2wavelength design frequency, based on distance from listening position to RPG), XPS/extruded polystyrene foam can be substituted for wood and can be cut into appropriate lengths with a simple hot knife. XPS-based RPGs make wall/ceiling mounting that much more viable (and safer) than 100s of lbs of wood to deal with.
if you have requirements for significant sq area coverage, it's wise to obtain (or find a friend with) a CNC machine and program the number pattern (MLS/pseudo-random) to construct 2-dimensional binary amplitude gratings. they are highly beneficial due to their mid/HF diffusion and increased LF absorption (than a simple porous absorber would allow). they lack temporal dispersion (since no phase shift as it is a planar surface vs variable depth wells), but that also relaxes the minimum seating distance limitation. you can cover entire boundaries with these and sufficiently damp the high-gain destructive specular reflections to be lower in gain and thus not cued on by the brain for localization, imaging, etc. - all while maintaining mid/HF diffused energy within the room to alleviate the common suckout feel as when one erroneously applies too much broadband porous absorption in the bounded space. with access to a CNC, you can easily output the printed membranes and simply use to cover the porous absorber. it makes the process quite benign. much more useful for multi-channel/surround (since there are so many active sources and thus so many "reflection points" that must be addressed for such a wide listening area) - where-as RPGs (1-dimensional, with the wells oriented vertically such that diffraction lobes develop in the horizontal plane / for lateral reflections) are more suited for 2ch stereo reproduction environments.