rotatable handle 1010 is operably connected to a retainer 1050 by way of a shaft 1020 retainer 1050 is configured to function in conjunction with a pressure plate 1040 to apply a mechanical clamping force on the track 600 when in an engaged or locked configuration according to the first embodiment of the present invention as shown in fig2 a plurality of projections 1090 are configured to extend from a bottom surface 1041 of pressure plate 1040 preferably the projections 1090 extend from a region located generally in the interior of the bottom surface 1041 of pressure plate 1040 in this configuration the projections 1090 may be spaced from one or both of the ends 1042 and 1044 of pressure plate 1040 and from one or both of sides 1046 and 1048 of pressure plate 1040 or a plurality of combinations thereof it can be appreciated that spacing the projections 1090 an approximately equal distance on opposite sides of shaft 1020 will ensure an equal load distribution across the bottom surface of the pressure plate 1040 preferably projections 1090 are positioned in a configuration shown in fig2 the projections 1090 may include four periphery portions 1091 formed in a shape conforming to that of scallops 1095 in track 600 to promote.