Bumper? Or tail light panel? All the panels I've seen use carriage bolts through the bodywork. That gives the smooth head presented to the outside and doesn't look so "kit-car-ish". You can get them in chrome, too. Both of my old cars used 3/8" bolts if I remember right, and just 2 to hold that panel on. Alternately, just fiberglass a block of wood behind the panel and screw in a dowel hanger (screw thread one end, bolt thread the other), then you can have a "fastenerless" rear panel.