Did you take the frame and monitor, or just the monitor?I can't help with the glass. I took mine to a hobby shop and had them cut a piece of museum plex for me.
Centering is a pain no matter what. Just trial and error and honestly takes the longest to get done.
I taped the plex to the inside of the frame to keep it from shifting. I used gorilla tape for that. I didn't tape down the monitor. Just connected it to the bracket and the bracket to the frame.