People have lots of solutions.
Personally I push it up against the front door, and on the right (where I have a bit of extra space on my crumbtray anyway) I push in a honeycomb holddown pin. There is a tiny bit of wiggle with that, so if you’re cutting right up to the edges, then making it exactly the width of your tray so it sits inside the plastic edges might work even better.
There’s also a whole grid system that removes the need for a jig, but I’ve never actually done it so someone else would need to chime in!