Abstract: A structure is described that includes a central channel with first and second ledges on a first and second side, respectively, of the central channel. The ledges include a first cutout in the first ledge and a second cutout in the second ledge aligned with the first cutout. The structure also includes a first side channel and a second side channel that each include an outer wall. The outer wall on the first side channel includes a first outer wall cutout that is aligned with the first cutout and the outer wall on the second side channel includes a second outer wall cutout that is aligned with the second cutout.