grounding homemade light fixture
My first suggestion would be to get 18 AWG fixture wire (TFFN/AWM) instead of lamp cord -- portable cords aren't rated for use within fixtures, and I'd cite one inside an originally permanently wired luminaire as a 400.8 violation.  As to grounding the actual fixture -- I'd use a standard conduit grounding bushing such as this one for 1/2" pipe where the fixture attaches to the ceiling box (presumably through a conduit knockout faceplate).