The easiest, fastest and most repeatable way I have discovered of doing something is cutting out a jig and using the 12x20 artboard. And as Jules said, put the reverse engraving directly over the front engraving just as a different color.
@Jules’s gives the blow by blow.
To be able to repeat this without having to cut a jig every time. start with a piece of cardboard or wood with two sides that make an exact square corner. Align the bottom edge of the cardboard to the front garage door. Use the right side of the crumb tray where the honeycomb meets the plastic tray as the other registration line. Cutout the spot for the card, drop in the aluminum and process front and back.
Also you can register to the back side of the Glowforge too on the top edge. I think that is how I did the clothespins.
@macphee just taped cardboard on the bed and then cutout the shape for the tongue depressors and did 8 at a time, repeatable. This works repeatable as long as you are using the 12x20 art bed and don’t move the jig between prints.