I highly, highly recommend AGAINST using smoked plexi over the monitor.
Older monitors have lighter colored tubes, and often have screen burn, and the smoked plexi helps to hide that. They look BAD without a tint over them.
New monitors already HAVE a tinted coating on them. Also, most new games don't have tint over the monitor, probably for this very reason. Putting the plexi over a perfectly good monitor just dims the picture and you'll have to crank up the brightness to compensate. Might as well just turn down the brightness and get the same effect!
I've been asked to give an opinion on this. For the record, this is strictly a matter of taste. But there are benefits to using the dark plexi.
As stated, screen burn and light colored phosphor can be hidden by dark plexi. But even with the newer tubes, just turning down the brightness doesn't do quite the same thing.
One feature of the dark plexi that I really, really like is the fact that screen borders can be completely eliminated. On a 27" monitor, a vertically oriented game just looks like a 19" vertical monitor is in the cab, not just a smaller screen that is pasted into the center of a larger horizontal one. If you use artwork graphics to fill in the edges, this may not appeal to you.
The reason this works is that a CRT can only achieve a certain level of "blackness". The scanning of the electron gun is still happening in the black areas, and a certain level of phosphor excitation is still occurring. The dark tinted overlay will make the black levels nearly absolute and by increasing the contrast, a bright image is still achieved. The dark tinted glass of the newer monitors helps a lot, but they aren't designed to completely negate the background like this method does. The result is also very comfortable for "in your face" viewing for long periods of time.
As I said, it's a matter of taste. Those who are using this method seem to like it. OSCAR liked it after trying it for himself based on my recommendations. And I personally like it enough to make it part of my current setup, as well as to make it part of my plans for future units. So if you are at all interested in what can be achieved with it, give it a shot. It's probably the only way you'll know

RandyT