Okay... I went back out to the HT room and took some more measurements. Using an old LD player up on its side, it is 16" deep by 17" wide. I could pull it 2" out from the wall for a full 18" in depth and the 17" was the absolute max height before the recliner hit it. I can go up to 96" on the width max.
So internal dimensions could be 12.75" high x 16.5" deep x 94.5" wide.
Using 3/4" MDF:
