So, this morning, I bought this scribe for £5.01:

This is to get at it's scribe tool, which is 2mm in diameter and 24mm long:

I was expecting that it would be flat on the other side, such as in a different product:

This means I'd probably need a design that uses a grub screw. I got this after measuring the angle of the £100 tool and then looking for anything with a diameter less than 4mm that has a similar point.

I then was reading through past logs, and one time I tried designing a PCB load cell. It seems, however, that I never tried simulating an aluminium PCB, which could cost under £5 from JLCPCB. The results with or without SMD resistors isn't that promising, but at least the deflection is less than 0.1mm:

The bigger issue is the turning radius of the sliding medium. I found the small section of 2.4mm chain and, unlike the 3.2mm stuff, it can actually go around 7.5mm diameter bends just fine.

Another consideration is that it actually turns out that my fingers easily use up the entire length of this new 52mm solution, though the utilisation would be better if the motor sprocket didn't force the Tetrinsics to be so close to the edge:

The orange section is the length of chain that would work for a pull-trigger style input, like what I'm doing on Finger5 for the shift key. Another way to put it, the boundary of the blue and orange boxes is where my fingers naturally rest. As long as all fingers can cover over half of the length (26mm), it should be fine. The earliest Tetrinsics designs only had 24 or 32mm of active area anyway. 52mm is about the spacing between 3 rows on a traditional keyboard.