Here is a video of a guy cutting OC 703 panels to make a corner stack Bass absorber. With a creative template, you could make anyshape you want.
http://video.google.com/videoplay?docid=3067566258802658704
This is what I did for the two front corners of my room. But my corner traps are 7' tall. It made a huge difference for me. I had the SMS-1 active before I built the traps and could not achieve a flat response. With the traps installed, it is reasonably flat and sounds tight and smooth. This is not an expensive project, either.