I went to Orchard Supply and picked up some knurled inserts and screws. You can find them in the bins where automotive, closet, window, grommets, etc. are located. They look like this (1/4 ~ 3/8"):
Screws should be about 1" long if I remember correctly. Test them out with the inserts and match the threads.
Make sure the holes are smaller than the inserts. Use a pair of pliers to sink them in the bumper snug. This method worked great for me and the skid plate is totally secure. I'm confident it will not come loose (unlike many other members here who used the OEM hardware and have problems)
You can use the decorative caps with the new inserts:
Good luck on the install.