Looks like something like this may help:
bumper protector
It's an aluminum protector that is wrapped in front with black rubber. It runs along the underside of the bumper on my s2k and does an excellent job of taking those kind of scrapes instead of the bumper.
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'02 Honda S2000
Elise ordered 1/13/2005, expected mid June, arrived unexpectedly on 4/29, I had to pass...