on the mounting: you've already finished and installed your admin panel - often a slot routed in the top of that holds the bottom edge of the glass. you could cut an L shaped piece to mount behind it and hold the face of the plexi tight to the back of the admin panel. that would take care of the bottom. the top is easy, just mount some blocks behind the plexi that push it tight to the back edge of the speaker panel. then, you slide it in and the top and drop it down into the bottom slot.
a good lcd monitor mount from Knievel

On the monitor angle, I would recommend that you take off the stand and use the vesa mount points on the back of the LCD to hard mount it to the cab, at the same angle as your plexi. you don't want your display loose inside the cabinet.