Hi MRAKGR, shell in MoI works best either on a solid or for thickening an open surface it's best if it's a smooth surface.

You could try something like Edit > Separate to break it into separate surfaces and then use Shell to thicken all of those but like Burr writes above on a real box there is a little bit of empty space between flaps which might help if you had that here as well.

- Michael