To be honest, I don't know that there really is such a thing. Lexan is fairly scratch resistant and incredibly strong, but you can scratch any plastic.
Lexan might be your best bet. Just be careful when you wash it. Use only soap and water, hose it off really well before touching it with the sponge, and be gentle with the soap sponge when you do wash it off.
If you want something tough, check out 3M anti chip tape at an autobody paint supplier. It is extremely tough. Racers use it to protect the leading edge of their bodywork and wings. It is ofter refered to as helicopter tape as it was invented to protect the leading edge of copter blades in nam. There is no plastic that won't scratch. There are anti scratch coatings, like they put on plastic eyeglass lenses, but I have no idea if it is available for DIY use.
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