We have the same problem everyone else does: trying not to scratch the fender while leaning over the engine bay. We've got commercial fender protectors, of course, but they sit folded up in the garage and over time get impregnated with dirt and grit. Also, we tend to forget to pull them out when a "simple" job needs to be performed.
That's why we think there's a better way to go about protecting the fenders. The answer for us is binder clips (spring-type paper clips)! Yes, these little black spring clips are terrific for use in protecting your fenders - or anywhere else for that matter.
Instead of a fender protector we use an old blanket or bedspread. We fold it over and lay it on the fender, clipping it to the flange around the hood opening area using the binder clips. It's quick and easy and offers the advantage of being able to shake the dust and dirt out of the cloth, or even washing it when it's really dirty.