I created my own bezel from poster board, but over time it has warped, so I'm going to do it again using matteboard. Essentially, I just took long strips of paper, trimming them repeatedly until they matched the monitor curve, and once they did I used them as a pattern to cut out the posterboard.
For a 21" PC monitor, a Happ 19" bezel should be a close fit, but you may need to trim it as the curvature of the monitor won't quite match an arcade monitor.
--Chris