Awesome! The 3 point fiducial ruler would be incredibly useful for many things.
An idea for a 0,0 corner outside of the L shaped ruler wothout having to add anything:
Just remove enough plastic to have a small gap/slot (maybe 1 inch) in the side rails of the tray which would allow you to lay down a ruler perpendicularly across the tray to act as the restraining guide on the y axis. Then all you would need is a standard 1" wide ruler
I know it’s a bit late in the game for that, as you’ve probably finalized all that stuff, but would be an easy fix going forward if you ever did decide to change.