Wow, procrastination certainly has it’s perks. Like others have said, I had a very similar idea to your base ruler, but I couldn’t decide if the magnets would produce a strong enough grip to the tray and considered also adding key holes into the ruler to slot something through both the ruler and the crumb tray for added solidarity. In hindsight, I already had the answer in these: Honeycomb bed holdown pins as they could hold the ruler in place with the added benefit of the magnets.
Thanks a ton, this will help with the projects that I’ve wanted to spray paint after an engrave, but before a cut!