@vogelrnws nice to se the layout. That battery holder looks pretty solid and as you said 2 *12V * 9Ah gives 216WHr of capacity. a LiIon at 4.4AH @ 4V is 16Ah.
Its great to have a discussion about the mechanical side, what works, what doesn’t work so well.
Seems like on of the challenges is that uSD is right up against the side of the cabinet – doesn’t appear like it can be easily removed. Would require the Teensy 3.5 to be pulled first.
I wonder also if there is any protection for static/lightening on the instrument leads. That’s a problem in certain parts of the country.
The DIN rails take some thinking about. The issue as I see it is how to do assembly simple and reliably. If the screws are inserted from the front, and require a nut at the back to hold it, it can become a challenge for replacement in the field.
https://www.mcmaster.com/din-rails/ shows some of the options for clips that might solve that.
For the Mayfly, in the Arduino style of a low cost environment, I’ve been using a plastic polycarbonate bar, and configuring it for the mayfly holes. Here is a write up .. Bud Ptq 11050 Enclosure