Three-dimensional structure is provided that includes an internal structure and an external structure configured to receive the internal structure. The internal structure includes a plurality of walls forming an accommodation opening and a magnetic element configured to be affixed or located within on the plurality of walls. The external structure includes a set of two surfaces having a first configuration of protrusions where the two of two surfaces are adjoining. Another set of two surfaces have a second configuration of protrusions, where the surfaces form at least one opening configured to allow insertion and removal of the internal structure. The set of two surfaces and the another set of two surfaces are configured for mating engagement with another external structure having substantially the same configurations, where the first configuration and the second configuration are configured to couple to each other.