Here's the modification I did today to turn the TwinMate into a Zone System meter.
I put a screen protector over the front. Moved the calculator dial a stop at a time and marked the tip of the green arrow across the scale. Then I cut a sheet of brass into a comb to range from Zone II to Zone VIII. At left and right, there is barely room for 3 stops but marks for Zone II and VIII are necessary, so those two end marks are cut smaller. The VIII mark is "less accurately" cut than the rest - that's just a workmanship problem, not a design feature. I was very impatient, didn't wait for glue or paint to dry - so you can see it's rough. No Zone Roman numerals, you have to count them out.
I glued "on top" of the green pointer and with rough side up, so the modification won't interfere with the meter needle.