You could get a sheet of white Masonite and cut it to fit on top of the window sill.

Do a google search for ..... marble window sills.... you'll find places that will custom make them. We recently built and got marble window sills through our local lumber store. They are fabulous. No worries about water or heat damage. You could also try a solid surface such as corian.

Just wanted to add that if you are looking for somthing temporary or "low budget" find a siding or window installer that might have some poly coil on hand.

It is PVC based and can be bent so you could use it as a guard on your window sills.

GulfBreeze, good idea and you just gave me another idea, just a scrap piece of PVC siding cut to fit. Either way, you can mold it to the sill or close the window down on it to hold it in place.

Dog slippers/shoes

piece of PVC will do the work

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You could apply ceramic tile. We did this with one basement window sill that was constantly getting wrecked - worked well! With all the sizes, patterns, and edge finishing available you might be able to come up with something that looks pretty nice.