How about this site for reference:
http://www.mhsoft.nl/diffusor.asp#calcul
Also, the Auralex metrofusers aren't hugely expensive, but they need to be covered somehow since they are made of Styrofoam and are butt ugly on their own. They are not based on the formula above though. They have varied widths as well as depths.
http://www.auralex.com/sound_diffusor_metrofusor/sound_diffusor_metrofusor.asp
Lastly, simply PVC tubes might be an option, but there is no real data to support them. Perhaps look up "room lenses" and you'll find something. The room lenses could also be seen as helmholtz resonators too...but I am no acoustician - that I am certain about!
http://www.mhsoft.nl/diffusor.asp#calcul
Also, the Auralex metrofusers aren't hugely expensive, but they need to be covered somehow since they are made of Styrofoam and are butt ugly on their own. They are not based on the formula above though. They have varied widths as well as depths.
http://www.auralex.com/sound_diffusor_metrofusor/sound_diffusor_metrofusor.asp
Lastly, simply PVC tubes might be an option, but there is no real data to support them. Perhaps look up "room lenses" and you'll find something. The room lenses could also be seen as helmholtz resonators too...but I am no acoustician - that I am certain about!