The best solution is an interlocking structure (dovetail joint). ABS should be gluable with acetone (I havent tried it). If the contact area is small and has long contact edges you can plastic weld the parts together by using a thin pre-extruded plastic thread an an old or covered temperature regulatable soldering iron. Also there is the possibility of using short pieces of filament as dowels. Heated nails in preprinted slightly to small holes can work too. If you glue make sure there is no lever effect possible.