Initially, I was going to cut arcs out of 2x6 dimensional lumber, but my jigsaw (and my jigsaw technique) proved woefully inadequate to the task. As I tried to follow the curve I had drawn, the saw blade bent out, leaving an unacceptably uneven cut.
Not wanting to purchase a scroll saw (and not sure it would be adequate anyway!) I decided to improvise. Using one metric shit-ton of geometry and trigonometry, I figured out what lengths I'd need to cut small pieces of wood to approximate an arc from a circle with a 4-foot radius. I cut my 2x6's into pieces 1.75" high, cut them a million more times, and assembled this frame:
You can see there's one 2x4 down the middle, with a little pyramid-like stack of boards I cut from the 2x6's. I also cut a 1x4 strip with a 45-degree bevel on one edge; that's the piece on the long side of the frame.
This design has the side benefit of making the whole rig somewhat lighter as well.
Here's a shot of the strip that will hold down a piece of hardboard on either side of the diffuser:
And here's what happens when you select a 1x4 with a great big knot in it:
However, with a backing board and several deck screws, it's like the Good Lord never put that knot in there. Ahem. Plus, it will make a great place to hang microphone cables when it's done! Yeah!!
Here's a stack of the little boards before I painstakingly screwed them all into place; it's slightly easier to see the individual boards when they're just loosely stacked on top of each other:
I proceed to painstakingly construct a total of four of these little piles of joy, one for each rib:
...and then put the pre-cut piece of hardboard in place:
After tucking in the left edge, here's the finished product. Note how the stack of wood supports the curve just so. It's so very close to perfect that I'm beside myself with pride...and we know what that comes before.
Here's the bare wall that's causing me so much grief next to my drumkit. Notice that the ceiling is EIGHT FEET HIGH, but that the rafters are NOT EIGHT FEET OFF THE GROUND. You've heard the expression "Measure twice, cut once?" Well, yours truly, who up until now you might have thought was pretty clever what with those stacks of wood that create supports for a curve of a predetermined radius, NEGLECTED TO MEASURE ONCE. My diffuser is eight feet tall; my rafters will only accommodate objects less than 7 feet tall.
Here's what it looks like when you realize you have to cut a foot off the top of your fucking perfect polycylindrical diffuser:
But it's easier to cut off the top of it than you might think, and all's well that ends well. Here's the finished product, in place and making my drums sound wicket ahsum:
Now it's time to build a 2-foot radius model! I'll post a photo of that when it's done, if I manage not to kill myself with the power tools involved.