If you happen to have a LCD monitor that can be physically turned by 90 degrees, turn it by 90 degrees, fire up mame, start a vertical shmup and rotate the display by 90 degrees in the video settings. You'll find that playing a vertical shmup on a vertically oriented monitor makes a HUGE difference. Of course, horizontal games will get a smaller screen size. It comes down to which games you prefer most.
Alternatively, you could come up with a solution to make the monitor rotatable (which will probably a bit complicated), or you could build a cocktail cab with two sets of controls, one CP at the horizontal side and one on the vertical side.