FlashForge/PowerSpec glass-bed z-axis shim

The Flashforge Creator X (and re-badged PowerSpec 3D X) printer ships with an aluminum bed. Some owners prefer a removable glass bed for ease of cleaning and setup.
Using a glass bed (I have both 1/8" and 1/4" glass) causes the limit of z-axis travel to be offset, such that the limit switch won't engage until the extruder has tried to punch through 1/4" or so of glass. To correct this, you need a shim to raise the effective height of the bed support to match the increase caused by the glass.