I covered the metal in the area with clear packing tape, then buttered up the inside of the fairing with a mix of epoxy/cabosil/micro. Put the fairing in place and screw it down-----little bees wax on the screw threads so the epoxy goo doesnt stick.
Smooth out any material that squeezes out, but leave enough for final sanding to shape.
After it hardens, remove the fairing and finish shaping/sanding.