Abstract: A modular support assembly for retaining accessory adaptors on structural members having circular or rectangular cross sections, comprising a first clamp member and a second clamp member formed so as to accommodate an accessory adaptor and the structural member there between. An accessory adaptor is formed with a shoe member and a cantilevered portion, the shoe portion being slid into mating engagement with first claim member by insertion of a mating protrusion into a channel of first clamp member. First clamping member, including inserted shoe portion, and second clamping member are then placed on opposite sides of the structural member at a point where attachment is desired. Actuating knobs are then repeatedly turned, causing retaining devices to threadedly engage threaded recesses. Operation in this manner tightens the support assembly and thereby completes the installation of the modular support assembly.