Abstract: Kit for assembling a rectangular rigid support structure including a pair of vertical support rails and an upper spacer and a base spacer. The two spacers are rigidly attachable to and space apart the pair of vertical support rails. The spacers have spacer ends with integrally formed inter-engagement arrangements for mechanical inter-engagement with and attachment to the support rails. The inter-engagement arrangements include one or more integrally formed projections intended for insertion into the support rails. The spacers in the kit each have two pieces that can be rigidly assembled together.