Well a 21 inch PC monitor would probably work fine as far as screen size goes, but I dont think it would fit well in the cabinet because of the actual size of the monitor.
When I tried the decased 19 inch monitor I was going to have to remove the coin mechs from my coin door to make it fit. On top of that, the thing didn't really fit right against the bezel arcade depot sells and was a little small on all sides.
Plus there is the cost factor. I could probably buy a Happs Vision Pro 19 inch monitor and an arcadevga card for cheaper than a 21 inch refurbished monitor.
Thanks for the kinds words,
-Dweebs