ABS specific glue, ( not ABS combo glue) from Home Depot, or Lowes, works well, along with drilling 2 small #6 screw holes to insure it's adhesion, after abrading and cleaning both mating surfaces well, using a solvent like BrakeKleen.
second alternative is strips of 3M double sided black trim strip mounting tape, done in multiple strips, at least 4 or five, side to side, and clamped with wood blocks overnight to insure complete adhesion and compression....
these work