A mounting base mechanism for releasably securing an accessory device to a mechanical structure having opposed oppositely angulated clamp surfaces. The mounting base mechanism defines a first clamp surface having clamping engagement with one of the angulated clamp surfaces. A clamp support structure projects laterally from the mounting base mechanism and provides support for a clamp actuating mechanism having a clamping plate. A retainer device supported by the clamp support structure is operative to move the clamping plate substantially linearly for clamping engagement of the clamp plate with the other of the oppositely angulated clamp surfaces. A keeper member prevents inadvertent separation of the retainer and clamping plate from the clamp support structure.