These overlays we are producing are for a portrait mode PC monitor or TV, in particular large format displays. I have an arcade cabinet set up with a 40" 4K Vizio TV, though I think a 43" 4K TV is the better size for this format.
This style has become more popular recently, with vendors offering pre-fab cabinets for purchase. There are also plans available. But a lot of people still just pick up a large TV and connect it to their regular PC. You can get a 43" 4K TV for <$250 now, so it's not that expensive to do.
Thoggo, who works on these overlays with me, and put our web site together also created a YouTube video explaining the history of MAME cabinets and how these vertical overlays evolved. You can see that here: Intro To the Home Arcade Experience
Here's a shot of my cabinet, which is the 40" model RecRoomMasters was producing last year. They currently only offer 32" models, which I personally feel are just too small to do the format justice.
[ATTACHED IMAGE - CLICK FOR FULL SIZE]
Our vertical MAME overlays: http://verticalarcade.com