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.
I thought for sure someone would be drooling over this.
There are several iPhone apps that work very well and are accurate enough for most old model cameras (like my Agfa Clack).
Otherwise, get a nice small Gossen Digisix for a small price 2nd hand. Works very well and will fit on your camera:
* I'm an analogue enthusiast, trying not to fall into the digital abyss.
* My favorite cameras: Nikon S2, Hasselblad SWC, Leica SL, Leica M7, Russian FKD 18x24, Bronica SQ-B and RF645, Rolleiflex T, Nikon F4s, Olympus Pen FT, Agfa Clack and my pinhole cameras.