Adding those MAME buttons interfered with the open space I had on the top of the controller cabinet so I modded up an old keyboard tray I had sitting on a shelf. Had to move the 3X1" forward from where I'd installed it a couple of days ago, but aside from needing to be stained at some point, the tray seems to work as advertised.
I can't imagine too many other possible additions to this, but I do kinda have my eye on an arcade "spinner" from the US.