Acoustical Frontier's offering for rear diffusion:
Vicoustic Multifuser DC2 is a 5.5" deep, two dimensional diffuser constructed from expanded polystyrene
A great value high performance 2D diffuser at 1/3rd the cost of the similar RPG Skyline. 2D diffusion is typically used on the front wall between the speakers and towards the rear of the room to improve clarity, spaciousness and envelopment. These products are acoustically quite benign and can be placed close to the listener, unlike deep QRDs. They can also be mounted on top of other acoustical panels and bass traps.
2D diffuser - scatters both vertically and horizontally
24" x 24" (2' x 2') nominal size
Expanded polystyrene construction
Available in black, white or grey
Mount to walls or ceiling using velcro or construction adhesive
Supposedly similar in performance to RPG Skyline Diffuser.
Here is a photo:
If anyone has information for 2D or 3D diffusers for ceiling application would appreciate it. Still haven't decided on which diffuser yet.