One last point to finalize the design:

4) Would you like a remote display bar that you can glue over your panel, and place the instrument box somewhere else?

The display bar should be about 60x10x4 mm, including the calibration button and light sensor.

The present box is 60x70x15 mm and as you can see on the pictures it fits to the side of a 57 mm instrument, in a 80 mm hole.

I think I will design the new PCB to allow a detachable display bar anyway.

I'm still open to any proposition...