A mechanism 144 for removably fixing tower 42 to crossbar 40 of first end support 14. Tower 142 includes an end surface 146 into which protuberances 148 and 150 extend. Upper surface 148 of crossbar 40 is formed with a first opening 158 having a recess 160 with an undercut hollow 162 shown in phantom on FIG. 12. A second opening 164 is found on surface 148 of crossbar 40 and includes a spring loaded plunger 166. Plunger 166 is intended to engage a slot 152 of protuberance 148. Once protuberance 150 is placed in opening 158, boss 154 rides in undercut hollow 162 and tower 42 is swung into place such that protuberance 148 of tower 142 enters second opening 164 and is held in place by spring biased plunger 166. As such, tower 42 engages and fits into crossbar 40 in a bayonet connection fashion.
