Using the Farnell base is so tempting; that's a really good idea. I wonder, could laser cutting give the features on the face of the buttons? Probably not the texture, but the Left and Right curves and the Up dome shape?

I wonder if a similar approach could be used for a knob. Finding matching shafts and sticking on laser cut pieces. But same there, can it give the dipped in middle and little bumps on the outside?

My biggest fear of multi-piece buttons has been the glue. What kind could be used, would it be provided with the buttons upon sale, and, most importantly, would a layer (or two) of glue obscure the transmission of light? And also, I've never had good results glueing flat surfaces to eathother. Would that post a problem with this redesign?