For what it’s worth, Paul McGowan prefers diffusers to absorbers; the former mix reflections of all frequencies equally, while the latter absorb different frequencies to different degrees. McGowan also explains that the reason “first reflections” cause problems for imaging and soundstage is that they arrive at the ear “too quick for the brain to sort them out as delayed copies,” so they muddle the spatial effects of the perception of the direct sound.
It's easy to make a diffuser. Go to Home Depot and buy a long, square wood pole. Using a skill saw, cut it into lots of small cubes with angled cuts. The angles don't need to be precise or the same, nor do the cubes need to be the same height. In fact, the diffusion pattern is actually enhanced by irregularly sized and shaped pieces. Softer woods absorb more than harder woods, but either will work. Then glue them, flat side down, onto a board of the size you need. If you want to absorb some lower frequencies, glue them onto an acoustic ceiling panel (also available at places like Home Depot). Works well and looks good, too.