The last couple times I drove my car I've been cringing at all the rocks I hear hitting the underside of the car, and I'm not driving on any dirt rally stages! I've got sticky street tires, fiberglass fenders, and no sound deadening so that might make it sound worse than it is, but I've also realized that my 911R taillights are right in the path of spray when I drive through puddles, which can't be good for their longevity. Has anyone made any DIY fender liners to protect fenders from spider cracks in the paint and keep water from getting onto the electrics? I've heard of racers putting some wetsuit type foam on the inside of the fenders but I'm wondering how thick it needs to be (since I've got little clearance at full bump) and whether adhesive backed foam would stick well enough. For the water spray I think I'd basically want a mudflap but one inside the fender like the fender liner on a modern car, but I haven't come up with an elegant way to mount it.

Any suggestions?