Using a small piece of bar stock the same width of the plastic piece, drill 2 holes, one the same size as the screw in vent door. Give the short bar stock a bend in middle with the same angle as the ramp on the plastic piece. Then bolt the bar stock over the plastic piece with the small plastic tab that was broken off still in place, this maintains the angle and keeps it flush, then sink a second small bolt a course thread screw into the body of the plastic piece.