Is the plastic channel under the side rails there on both sides?

Is the width of the glass the correct size?

If both those answers are "yes" then you probably should try this to snug it up:

http://pinballpro.net/shop/anti-rattle-tape-ar-1/

Anti-rattle tape?

I had to use it when I owned Star Trek Next Generation. I put in upgraded speakers and subwoofer. The glass would rattle in the channel. Had to put the special tape on the edges to stop it from rattling.

Stand against front of the game and block the glass with your torso as you release and lift the lockbar. Then rest a palm on or in front of the glass and put the lockbar down. Then slide the glass out.

I would try some adhesive grip tape you can get from Scheels, Dick's, etc. Cut a couple of inch long pieces and wrap around the edge of the glass on each side near the bottom. Hopefully that will do it, but if still loose add another layer of tape.

I would recommend always putting a hand on the glass and leave it there as you remove the lockbar. The glass stays in place or slides slowly on some of my games without the lockbar but I never take chances of it sliding out. That's what I was taught when I got my first game and makes sense to me. Even with tape or something, you should always make sure the glass is not going to move without you being ready.

As suggested by Vireland the pinball pro Anti rattle tape is the way to go not only for loose glass but as a protector on all glass.

I use it on all of my personal machines.