A kit for assembling a rectangular rigid support structure of a modular storage system, the kit comprising: (a) a pair of left and right vertical support rails (203, 204), each said vertical support rail having a generally U-shaped transverse cross section and including a front support rail wall, a rear support rail wall and a pair of opposite left and right support rail walls (513, 514) extending between said front support rail wall and said rear support rail wall for defining a longitudinal directed support rail channel having a longitudinal directed support rail slot formed in said front support rail wall, each said left support rail wall having a longitudinal directed channel including a front channel segment adjacent said front support rail wall, a rear channel segment adjacent said rear support rail wall and an intermediate channel segment connecting said front channel segment and said rear channel segment, each said right support rail wall having a longitudinal directed channel including a front channel segment adjacent said front support rail wall, a rear channel segment adjacent said rear support rail wall and an intermediate channel segment connecting said front channel segment and said rear channel segment; and (b) a pair of spacers (530, 530) for spacing apart said pair of left and right vertical support rails (203, 204) wherein each said spacer includes a pair of opposite spacer ends (531, 532) for mounting on said pair of left and right vertical support rails (203, 204), said pair of opposite spacer ends (531, 532) having a T-shaped cutaway (533, 536) for registration with said longitudinal directed channels of said pair of left and right vertical support rails (203, 204), characterized in that said pair of opposite spacer ends (531, 532) includes integrally formed inter-engagement arrangements for snug insertion into said pair of left and right vertical support rails (203, 204) on mounting a said spacer thereon. (a) a pair of left and right vertical support rails (203, 204), each said vertical support rail having a generally U-shaped transverse cross section and including a front support rail wall, a rear support rail wall and a pair of opposite left and right support rail walls (513, 514) extending between said front support rail wall and said rear support rail wall for defining a longitudinal directed support rail channel having a longitudinal directed support rail slot formed in said front support rail wall, each said left support rail wall having a longitudinal directed channel including a front channel segment adjacent said front support rail wall, a rear channel segment adjacent said rear support rail wall and an intermediate channel segment connecting said front channel segment and said rear channel segment, each said right support rail wall having a longitudinal directed channel including a front channel segment adjacent said front support rail wall, a rear channel segment adjacent said rear support rail wall and an intermediate channel segment connecting said front channel segment and said rear channel segment; and (b) a pair of spacers (530, 530) for spacing apart said pair of left and right vertical support rails (203, 204) wherein each said spacer includes a pair of opposite spacer ends (531, 532) for mounting on said pair of left and right vertical support rails (203, 204), said pair of opposite spacer ends (531, 532) having a T-shaped cutaway (533, 536) for registration with said longitudinal directed channels of said pair of left and right vertical support rails (203, 204), characterized in that said pair of opposite spacer ends (531, 532) includes integrally formed inter-engagement arrangements for snug insertion into said pair of left and right vertical support rails (203, 204) on mounting a said spacer thereon.