Make yourself a wide box with a height of 0.88" (or 0.5" shorter than your crumb tray), and set that aside. Make a pair of boxes that are exact crumb tray height.
remove tray.
Lay them side by side in the GF. Secure them with tape or magnets.
Tape cardboard to the outer two boxes. Cut your jig as if the vardboard is on the crumb tray, as it will be the exact height of the tray since it’s on the two tray height boxes you make.
Place your items on the center larger box. Set the height to be 0.6-0.5, since the box is 0.5 lower than the tray. Secure the items how you want to. I use blue tack, but double sided tape might work or whatever.
Engrave away.
I used this method on my rat skull engraving, and have the boxes hanging around. I’ll get you a pic.