Ed,
The glass panel was no longer available the last time mine broke (it was the second time), but BCS had the Plexiglas replacement in stock. The down side to the Plexiglas version is that it scratches fairly easily, so it does not stay as good looking as the glass version, but it will not break.
To replace it when you get the new panel, remove the plastic strip that the panel slides on. Clean out all of the glass pieces, insert the new panel, and with it in the open position, slide the plastic strip back in under the new panel. install the knob, job done.
Gerald