Hang items without a C-Clamp adapter on the fixture.  This double clamp grabs both the hanging bar and the fixture.

Specifications:


Dimensions: 5.5" x 5.5" x 2" (14 cm x 14 cm x 5.1 cm)
Weight: 12.2 lbs. (1 kg)
Safety Cable Size: 1/8" x 36" (0.24 cm x 91 cm), 100 lb. work load (600 lb. test)
Minimum Size: 1.5" OD (4.2 cm) pipe
Maximum Size: 2.5" OD (4.8 cm) pipe
Safe Vertical Working Load: 500 lbs. (225 kg)