Well, on Mar 9th, I came up with the sliding button idea as well.
 I started to design its functionality... but got distracted and didnt finalize it.
 After drawing up an entire mechanism, I realized one important thing I had overlooked.
 Buttons should be close as possible together.  But if you make a slider out of mere steel or wood.. it will not have the strength to hold up to downward pressure, because the button-hole will make the edges too thin.
 I then came up with a fix, but have not yet drawn it out.  By using a "C" shaped piece of metal (or possible an L ), you can still cut the hole out but still maintain a lot of strength due to the vertical thickness.  Yet, it will still be fairly thin, keeping the buttons close together.
 To hold the thing in place, I was originally going for a pressure latch seen on some cabinet doors.  Strong Neo magnets might be just as good.  Use of a small nub to drag the thing externally... or use of a small hole which you then insert and use a 'key' to drag it.
Click to expand the drawing.
Key:
Light Blue = Plexiglass
Pink         = Sliding Black Bezel (covering holes / slots)
Dark Gray = Metal Plate
Shown were two methods:  Over and Under bezel methods.