For building a skyline diffusor, you may want to check out gearslutz.com and go to the studio building and acoustics forum and the sub-forum for bass traps, acoustic panels, etc. There is alot of DIY info about all types of acoustical treatment, including diffusors.
By the way, I recently put two GIK Acoustic diffusors behind each of my Magnepans. The two together measure 4 feet tall by 2 feet wide. I mounted them on the wall, which is about 5 feet behind the speakers. In my case, the diffusors behind the Maggies have made a significant improvement in the sound. In addition, I put a GIK Tri-Trap on the floor against the wall behind each Magnepan. Now I hear bass and kick drums like I never thought I would on a Magnepan.
By the way, I recently put two GIK Acoustic diffusors behind each of my Magnepans. The two together measure 4 feet tall by 2 feet wide. I mounted them on the wall, which is about 5 feet behind the speakers. In my case, the diffusors behind the Maggies have made a significant improvement in the sound. In addition, I put a GIK Tri-Trap on the floor against the wall behind each Magnepan. Now I hear bass and kick drums like I never thought I would on a Magnepan.