I'm working towards making a portable case for my 3D printer, But I want it to be (Relatively) future proof, so since I foresee wanting another (Or more) printers in the future, the case needs to be stackable to some degree. But, I don't want protruding parts. That means I will probably inset any trim/corner protectors. So, I've designed this one to sit flush (Routing out an area for it to sit into is necessary, I may make a jig for that.) But how to stack without sliding? Marbles. A typical marble is usually 1cm, so Each side of this 5mm thick part has a 1cm hemisphere cut out so a marble can be dropped in and things can be stacked up.