I built some polycylindrical diffusors many years ago and while they worked well
for reducing flutter echoes and reducing bass built-up, one needs a lot of them for
meaningful results. I used 1/8" tempered masonite stretched over curved
plywood members and were completely sealed (1/4" weather stripping) and
the inside was completely filled with fiberglass insulation.
Material cost was low, time consuming, and stretching the masonite around
the bulkheads was a little tricky.
