This thread got me to try an experiment. I went to the hardware store, got a pair of 3" test caps in the PVC pipe section,

http://images.hardwareandtools.com/T/6154587.jpg
sprayed on some plastic primer and Rustoleum Cobalt Blue Metallic paint, and hot glued them on. These are the results:
http://www.smartcarofamerica.com/gal...php?i=2250&c=8
http://www.smartcarofamerica.com/gal...php?i=2251&c=8
http://www.smartcarofamerica.com/gal...php?i=2249&c=8
I drilled drain holes in the bottom to take care of any water that might get in.
Just drove 100 miles with these on and they remained firmly in place.
Total cost was about $2.50 for both test caps and $6.00 for the paint (plenty of leftover for future projects).
This also raises the possibility of drilling a hole through the center of these caps and attaching 3 inch chrome/brushed bathtub overflow covers. I saw those at the store and figured they could be used as well for the "blingier" look, for those so inclined.