My two egg trays for the fridge take too much space when placed side by side. So why not stack each other? This thing allows easy access or removal of the upper container, which simply slides into the groove, while the lower container fits snugly and stabilizes the two things.
Of course, not all egg containers are the same, but there is a good chance that many of them come from the same factory. Check the basic dimensions based on the drawing on one of the photos (around 100mm (4") wide inside!) and probably print only one first to check.
In order not to unnecessarily need support material, I have the thing divided into two parts, which are joined together after the printing. Test that it snaps into the groove cleanly, it needs a bit of power and precision. Finally, both parts secured with a drop of super glue.
Mine are printed in NGEN Clear but that is just my prefered material here.
n/a NGEN or PETG
100% infill if clear material.