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I had to use a #8 screw to match the factory holed in the fascia. The holes for the halo lamps only accommodate a #6 size screw cover, so I ground down the head on the screws and it fits like a charm. The covers also have the little "lip" where the hinge. If you screw in fully for a firm fit, the cap gets pulled down into the Halo lamp hole at an odd angle. To prevent this, I put some washers behind each one so I could sock them tight and not worry about deflection or them cracking apart due to the angular force. Works like a charm.
What I said above is in relation to screws sized for the LED tail lamps. It may not apply correctly to the factory housings, so some trial and error will be in order.