I experimented with diffuser panels and also built some DIY room lenses (hemholtz resonators) with good results. However, the tube traps are far more effective other room treatment that I have tried. I looked at John Risch's tube trap design and figured that it would probably deliver the same results but was much harder to build, and probably uglier.
I found this idea on the web. THere is a link to the site in the construction section.