The marquee should rest on the wood. You should only need L brackets to keep it from falling forward. If you put L brackets behind it - it'll be dark on that spot when the light is on.
Here is a picture of my cabinet:

The actual size of the hole is 4" the size of the hole + the wood is 5.5". I'll make my marquee art 4" and put a 3/4" border around it. The L brackets I used were 3/4" and cover up the border. It looks like this once it's done.

I used screws to connect the L bracket to the machine. The L brackets hold the art which is sandwiched between 2 peices of plexi.

I used wood L brackets, but if I could do it again, I would use a metal one from Home Depot.