The Mugen Replica lip for the 01-02 coupes is made of ABS plastic. I too filled the holes on that lip, and I just used a bunch of ABS Cement (not sure what sleepercell/doctorX [the owner of the silver coupe] used). Works well, but no matter what you do that lip will be destroyed quite easily. Hence the black stripe on the lip in that photo. My car had the lip break in half at the exact same spot, hence the stripes.
It may look nice, but don't expect it to last too long. Any decent scrape on the ground and ABS lips brake.
This is the only photo I could find of mine, worked on it a bit more and actually made it to paint, but it died violently a few days later when it met a tire on the freeway.