Thanks for the enlightening discussion Thomas.
I'm thinking of not enclosing the sides at all. So, essentially, I'm imagining a curved 4'x8' plywood panel attached to the wall only on the short sides. From your explanations, the properties of the wood will change as the panel is curved, which is reasonable to assume. I imagine this becomes stiffer and drives the resonant frequency down. My hope is that it will be fairly broad, but at least be largely active in the second octave area. Would it be more effective/predictable to not bend the wood and simply make it a large flat-panel absorber?
The room is presently treated with ASC style Tube Traps which are in the front corners and between the speakers on the front wall. I have been experimenting with their placement by finding the points where I feel the most bass in the room, and putting a trap there. This helped a good deal to bring the fullness in the sound back.