personally, i think painting the back of the plexi looks way better than a separate mat behind it. i tried making a bezel out of black foam core i picked up at the craft store, but i just didn't like the way it looked. the nice glossy look of the painted plexi looks much more professional in my opinion. in fact, on my showcase cabinet, that is the way it was done from the factory. i do have a foam core bezel behind it, since i am using monitor that is a little smaller than the dead one that was in the cabinet. i didn't want to spray paint the existing one to mask it off smaller in case i ever decide to put a bigger monitor in. on my other cabinet, it isn't a big deal to pick up a new piece of plexi and make a new bezel if i change the monitor. (that is exactly what i did when i decided to change it from a horizontal mounted monitor to a vertical monitor. however on a showcase cabinet, the plexi is quite large and would be much more expensive to replace. (not to mention that there are slots cut in it in both sides for the speaker grills, i have no idea how i would re create those)
if you are using a silver TV (i have one in my showcase cabinet) i would highly recommend either removing the existing tv bezel and painting it, or covering it with electrical tape or something similar. i havent done this to mine yet and the silver edges can be seen...it is mildly annoying.