I got one for 19" screens. I cut it with a Dremel and cutting wheel.
The interior "hole" pissed me off. It was for a 19", but there's an inner "lip" along the inside "hole" that in the end blocks out over half an inch of viewing area. Since MAME already varies screen output for different games, with some having black borders, I wanted to make absolute best use of the available screen space, so I ended up cutting away that inner lip. The results aren't perfect (irregular cuts), but its not that noticable...