Alternatively, it is contemplated that, in other embodiments, the front and rear attachment members may be inverted such that only one of the rear left or rear right attachment members comprises a hook portion 1110 and the front right and front left attachment members 1100, 1200 does not comprise a hook portion 1110. The rack 10 is thus moved rightward or leftward upon being laterally positioned between the left and right vertical support units 104 depending on if the rear left attachment member has the hook portion or if the rear right attachment member has the hook portion.