Clear Plastic type inserts for covers

[GSL]

I had new covers made for my boat in Sant Carles 3 years ago, and the "window" parts of the covers have been badly damaged by the sand the comes in from the Sahara and sits on the covers.

It seems to eat into the windows and they are now totally opaque.

Does anybody have a recommendation for the best material that is resistant to this type of damage.

Thanks

Graham