JB Weld makes some epoxy putties that work really well for modifying grips (and fixing divots in forms...) Their KwikWood is usually available at local home centers and is the jelly roll style putty where you cut off a little, mix thoroughly by kneading with your fingers, then apply and let cure. Doesn't seem to shrink or crack or make as much of a mess as Bondo, but does take a little longer to set up.