I am always skeptical about imagining box like things. So what that I visualize containing that paper slip. It's in my body that this short memory is present. I can feel it. I cannot push it out of awareness by pretending I am putting in some kind of box. Memory manifests in my body. Your visualization is quite artificial. I know this is EMDR "tradition" that wants us to pretend the memory is placed in container. It it were so Id keep it there all the time and save therapy expenses.