I have over 20+ monitors in various sizes and types around. Here is the skinny on them.
PC monitors ARE NOT the same size as the equivalent arcade monitor. The only common arcade bezel to PC monitor translation that actually works right (fits the tube), is a decased 14" PC monitor matched up to a 13" arcade bezel.
If you aren't decasing your computer monitor, then forget it, no bezel is EVER going to fit it, because it ALREADY has the monitor housing where the bezel is supposed to go.
A 21" PC monitor is very close in size (slightly larger) to a 19" arcade monitor, and you MIGHT be able to modify a 19" bezel to fit. This involves carefull cutting with sharp knife.
Except for the aforementioned 14" pc monitors, arcade monitors and PC monitors are also curved differently, which is why it is VERY hard to match up the bezels, even if you can match up the tube size.
The best solution to PC monitor mounting is to do one of the following.
Decase the monitor, mount it on a wooden board that is cut to closely follow the size of the tube. Paint that board black, add smoked glass or smoked plexi. Alternately, you can do the same mount job, and then carefully cut black posterboard or rubber floormat to make a flat "bezel" (which is best hidden with smoked plexi).
Pull the front part of the monitor case off, paint it black with black engine paint or black BBQ grill paint (you don't HAVE to use that specialty stuff, but at least then you will be sure that your paint isn't going to get wet again once the monitor warms up. Put that back on the monitor, and then cut posterboard to cover the outer part of the monitor casing, then cover with smoked plexi.
In all cases, smoked plexi is the key when you want a clean look out of a PC monitor or television.
Also, you can probably get smoked plexi or glass local for as cheap as a REAL bezel would be anyway. I just ordered a piece of thick smoked glass (tempered glass too, not the off the shelf stuff from home depot) for a cabinet and it was only $20.