An adapter to detachably affix an implement comprises mutually engaging segments with cross-sectionally shaped structures. An upper segment for the base of a fuel tank bag is shaped similarly and in locking manner to a lower segment affixed to a fuel tank sealing lid. Both segments have frame-like shapes and are detachably assembled, one segment spanning the other from above or below in interlocking manner. The segments are fitted with aligned ends which can be assembled to one another with pairwise, diametrically opposite portions. The ends of the segment are hollow claws that partly enclose claw counterparts of the other segment. Ends of the upper segment are fitted with gripping ribs. A spring-loaded detent catch may snap using the ends into a borehole of a cog, thereby actuating a magnetic switch applying a low voltage to terminals. A cover mounted in vibration-damping manner may be fitted with an implement coupling.