You could only do this 1 of 2 ways.

The lens would be two pieces for each side of the volume control's arch.
Each piece would have to use tabs that stick out from the lens and be held in place under the edge of the openings in the fascia against something solid in the back (i.e. the glass of the display)
or else the edges of each lens would need a U shaped clip made into the edge which would still require recesses, allbeit smaller ones, in the back of the fascia, which would probably break fairly easily.

I think the best way, short of redesigning Empeg's design, is to mill out the recess in the back for the lens the way it is supposed to be.