You may have success with 2-D type diffusors but I would think about diffusion after treating the room with as many bass traps as you can build.
If you want any chance of good frequency response, treat all 4 corners with bass traps (floor to ceiling if possible) and the ceiling/wall junctions if you want even more improvement, but at the very least all 4 corners.
If you want any chance of good frequency response, treat all 4 corners with bass traps (floor to ceiling if possible) and the ceiling/wall junctions if you want even more improvement, but at the very least all 4 corners.