members 14 and 16 nuts 20 secure the threaded members 18 from the frame members 14 and 16 to their corresponding support members 17 in one embodiment the support members 14 and 16 include upright support sections 29 securing the corresponding upper support sections 14 b and 16 b to the associated support members 17 as illustrated in fig1 5 the support members 17 also have a plurality of tabs 22 extending from an inner bottom edge 24 of the support members 17 support member securing apertures 23 are located on a lower aft portion 25 of the support members 17 the support plate 28 has opposing sets of tab apertures 26 a 26 b 26 c and corresponding sets of support securing apertures 27 a 27 b 27 c each set of tab apertures 26 a 26 b 26 c are postioned for receipt of the tabs 22 from the support members 17 in various positions on the support base 28 the corresponding support apertures 27 a 27 b 27 c are positioned for aligning with the corresponding securing apertures 23 in the support members 17 in this manner the tabs 22 of the support members 17 can engage the selected set of tab apertures 26 a 26 b and 26 c of the support plate 28 in a pivotal manner once the tabs 22 have pivotatilty engaged the support plate 26 the support members 17.