It seems that the manufacturer of the replacement headlamp bucket (Magnum) leave out the crucial part - a captured nut, as seen in this photo (what a nuisance):
Martymexico's suggestion is a good one, although as I don't have a rivnut gizmo, I instead carved out a bit of the plastic with a dremel tool, and superglued a nut in place. Hopefully the little buggers stay put.
Attached Image(s)
