TROFAST inserts for small boxes (20x30x10cm)!
Look at IKEA_TROFAST SMALL_box insert if you prefer bigger boxes instead.
Optionally, print also lock. Once as it is and another one mirrored.
Stick locks into grooves in the bottom. Small boxes will be fixed in corner positions but you can still take them out.
Lock can be downloaded here:
https://www.printables.com/de/model/58061-ikea-trofast-box-insert/files
The author marked this model as their own original creation.