Still playing with the holder 3D model. I had to adjust the latch tab (again) a bit, to correctly engage with the inside of the calculator. I still struggle with the fact that I'm printing the holder "wrong". That is, I'm printing the outside surface of the holder as the bottom, and building upwards - this makes perfect sense for everything except the latch. The latch wants to be printed "sideways" so that the FDM lines are running perpendicular to the arc of motion. Unfortunately, on my printer, this would require a lot of support material and probably result in a really ugly holder.
I've tried printing with "soft PLA" (SPLA) and geez is that stuff icky. What a mess! I don't have a heated bed on my printer, so I can't print Nylon or PETG which would probably be much better.
My gut feeling is that, long term, once I've got the geometry of the holder worked out, I'll send off an order to a place that can generate very high quality prints in a nylon-like material and have a dozen or so made (I've got quite a few Ti-58 and TI-59 calculators, and maybe others will want some of these, also).