Hi Chris, there's no big secret, I'd say. Standard armored plexi crystals are availlable in several versions - at leat a major european supplier the types are something like, ATC (armored standard, chromium ring), ATCH (high domed), ATCR with modified ring (stepped underneith for more dial space), ATCB with wider ring (something like the railmaster, but not that wide).
Take a good caliper and measure the inside diameter (for the crystal seat) and order the plexy in that precise diameter. Increments are in 1/10th of a millimeter. Must be pressed in with proper tool - if unsure about proper measurement, order three of them +/- one tenth and you'll be sure to find the fitting one. They're not that expensive, having two left ove will not hurt
