Then again, there's always marble, granite, tile, slate, etc. You could even terrazzo the sills. Concrete?
Chess, maybe you don't have a materials dilemma. Maybe you have a canine behavior dilemma?
You could do 'em in wood and keep an 8' length of matching wood, already finished, around.
Cut fake top sills so that they fit really snuggly over the real sills, or 18 ga. 'em in. When they get trashed, flip 'em over. When that gets ruined, go cut off another length and replace.
Other solutions:
*Rubber-bottomed knit mittens
*Adopt a very short or disabled dog
*Learn to appreciate the design possibilities of barbed wire.
*Board up the windows, only 1/2 way though
A final serious solution: Paint. It's the easiest thing to quickly and completely repair.
Ulitmate solution arrived at by having four: Get over it. All of it. You live with a dog!
