I like using milk carton caps to hold food for my crested geckos, but they always manage to drag substrate into their food and tip it over.

So I made a rock style feeder that will prevent them from getting their food dirty. (there is also a version without rocks)

The ledge attaches to the glass with a suction cup that I just hot glued in with a bit of room for it to squish.

The main feeder was modeled in OnShape, and the rocks were modeled in Blender.