Another approach some have taken for custom built cabs is to route a slot at the top which is usually the speaker shelf under the marquee and a similar slot in the bottom ledge.
You make the slot slightly deeper in the top and cut the glass or plexi height in such a way so that when fitting into place you can lift it up into the top slot then set it in place in the lower slot.
I didn't have the forethought when constructing mine to plan for this need so was going to just add a small frame around it or a piece at the top and bottom. However, my cab doesn't move around so I once I set it in place, I never added anything to hold it there, it's own weight and tight fit does that just fine.