I hope this makes sense. The pictures I'm going to post are not of my bearded dragon cage but of a large cage of the same design.
This cage is about 48" x 30" x 48" - LxWxH.

This is hard to explain, but on the back of the cage the top 8" or so is completely open. So the back panel of the cage is only 40" tall instead of 48". It stops 8" from the top of the cage.
Then I built a frame that runs from the upper lip of the cage to the top of the back panel. Again, this runs about 8" below the very top of the cage. Here is a picture of this frame. It is hard to tell in this picture but it is covered with screen. Keeps the crickets in and allows ventilation.

To access the lights on this cage or my bearded dragon cage I just slide the cage forward and reach into the area.
If you go with this design make sure you build it to house a variety of lights. When I built my Bearded Dragon cage there were MVB bulbs that could be mounted horizontally. I only built this false ceiling area tall enough to hold one of those. They did not work well and I did not leave enough room for vertical MVBs - something I now regret.
Also, leave room for a 48" light fixture, even if you don't think you'll need it. My next BD cage will have enough room for the following:
1) Vertically mounted, high wattage MVB.
2) A four tube 48" T8 fluorescent fixture. This can be tough since most fixtures are a bit longer than the width of standard plywood sheets. But it can be done.
3) Several incandescent fixtures.
Why so many? Simply for versatility and in anticipation of the next new thing in reptile lights.
I hope this helps.