We have a 31lx too and measured the cavity at 41 inches diagonally, 25 inches in height, 33 inches vertically. However that aspect ratio is not that of a standard 16x9 tv. So don't be fooled by thinking a 41 in will fit. Bottom line we could only fit a 32in.
Now I'm trying to decide how long a screw I can use and not go through the master bedroom wall.
Still looking for a post on screw length.