Chris, given how thin of a coat you are making with the epoxy there really isn't much you are going to do by making the epoxy itself tougher. My take on it is you can make a layer of cloth/resin that has good abrasion resistance characteristics because the flexible fibers in the resin don't break off every time they get abraded,,and the surface gets fuzzy. Or you can simply add another layer of glass, cheapest and most effective method, or s-glass which is pricey but will go almost as clear as the regular glass and it will be more abrasion resistant than regular glass (a 2ydx60" piece for $30?), a piece of 6oz s-glass will be very tough ontop of the regular 6oz. Seems to me that putting on thicker layers of epoxy goop is kind of like making an epoxy gel coat, so why not just do that? Still it won't be as effective as some kind of cloth. I think this is one of those no free lunch things, any additive that will increase the abrasion resistance of epoxy (cabosil, aluminum powder) will look ugly and be messy to sand out. I'd simply slap on another layer of 4oz glass for the simplest and lightest solution.