Actually, I just figured it out. Someone put a metal L bracket there to hold the glass in place, but it must have fallen off when I was transporting the cab. I found it in the bag of bolts and stuff I pulled from the cab when I cleaned it. I'll try putting it back in place. Thanks for the input.
The other problem: I'm sure I'll be opening it frequently to change games. That's why I usually don't have the control panel latched, so the control panel can move around somewhat while playing.