Give Warwick Harrison a call, he offers an option to fit some kind of wet suit (neoprene?) material up inside the guards. The idea is to prevent stones punching through the inner guards and dinting the paint work from beneath. Plus it probably offers decent sound deadening at the same time.
I did a quick google and it seems to come in a few colours and thicknesses...
http://www.paskal.com.au/products.as...&cat1=Neoprene