The initial plan was to get a 4:3 TFT monitor for this project, either a 19" or a 17".
Budget constraints meant it was best to go with the 19" TFT I already had kicking around, despite the fact that it's a widescreen unit. I've been playing around with MAME's settings and although it initially stretched across the entire screen, I've got it displaying in its native resolution. I've been looking into pinball emulation and if I decided to go this way a widescreen monitor would be a bonus.
The current plan is to use a bezel to mask off the unused parts of the screen so it appears to be a 4:3 screen anyway.
Images above show the monitor in its decased state from when I was measuring up and initial testing with full screen stretch on Dodonpachi. This is not the actual unit I'm using, just a filing cabinet we have that was the cause of my initial epiphany with this idea.
I'm going to cut out a section in the top of the unit and let the decased monitor sit in it, flush with the surface. A sheet of smoked glass will go on top, with the bezel, most likely black card, in between.