I didn't see anybody else's ideas, so I don't know if this is a repeat idea. I remember when I started this part of the cab, I didn't know how to make the plexi look nice. I tried applying a vinyl to the back side and it looked terrible. So here is how I did it.
I used 1/4" plexiglass from Lowe's. Once cut, I placed it over the monitor where it will sit. I taped the outside(side facing player) of the plexi around the monitor as a guide. I took the plexi out, taped the inside(the side facing the monitor) edge to edge with the tape on the outside. This allowed me to know where my edges of the monitor were. I also placed a round 1/2" sticker where the remote control receiver is. After taping, I painted the area outside of my tape with the same semi gloss black that I painted the cab with. It dried within an hour. I peeled the tape and sticker, and viola, from the front it looked like a perfect bezel. It was very easy because all I had to do was lap the paint on the back. You can't see any of the streaks on the front. Just nice and smooth. Now for the best part.
I couldn't find smoked plexiglass in my area, so I went to a local car detail shop and bought a 24 x 24" piece of window tint.(56% shade). It only cost $3 for it. How nice to finally find something cheap! The guy who sold it told me said the trick is to use soapy water on both the plexi and the tint. So I cut the size that I needed, peeled the clear paper off the tint, laid it on the inside of the plexi, used a small straight edge to work out any bubbles and the final product was excellent. Since the tint has a water based glue, it dried on its own and looks smooth as could be. With the water, it slid very easily and took all of two minutes to apply.
I have a 27" Daewoo flat screen with components and I am very happy with the look now. My plexi is very close to the monitor, otherwise, I feel that the posterboard trick to hide the area between would have been an awesome option.
Since this is my first contribution, I hope it helps another noob just getting started!
EDIT: I have attempted to make it more understandable.