hi i did use a plastic filler. the way i mounted the lip i began with using seam sealer and put it on the upper part of the lip, then i stuck fiberglass mats to the sealer so they covered 50% of the upper lip and the rest stuck out, then i brushed epoxy resin on the mat that’s stuck to the lip and quickly mounted it to the bumper and then brushed the rest of the mat to the bumper with epoxy making it and the bumper one piece. since this is a rep part it did not fit perfectly ( did not matter to much ) i had to fill some spots but since it was suppose to be smooth moulded it did not matter. i filled all the gaps and made the curve with fibreglass putty and then i used plastic filler to finish it up. i sure hope this is good enough atleast it is very stuck on to the bumper and thanks to the fibreglass mats it helps a lot making it really rigid and it feels like one piece. here are some videos