Derailleur Hanger Gauge Swivel Block
Print two of these 60 x 24 x 22 mm blocks, add 8 mm steel rod and an M10x1 bolt, and you have a DIY derailleur hanger alignment gauge. One block bolts to the hanger through a 10.5 mm bore and carries a radial arm rod. The second block slides along that arm and holds a short feeler rod parallel to the axle. Both rods lock with M3 thumbscrews in heat-set inserts, so you can compare the gap to the rim at four points and bend the hanger true without buying a $60 DAG.
tool bicycle derailleur-hanger alignment-gauge