Abstract: A horizontal base plate structure is provided having a central opening therein and a plurality of marginal portions spaced about the central opening and disposed in a plane slightly spaced below the portions of the base plate structure defining the central opening. Upright central post structure is supported from a central portion of the base plate structure and attaching structure is carried by the upper end portion of the central post structure for attachment to a vehicle frame. Also, anchor structure is carried by the base plate structure and operative through the central opening therein for engagement with and tightly drawing the central portion of the base plate structure downwardly relative to a floor anchor over which the base plate structure may be disposed.