I used to own a Thorens, and cleaned/polished the dustcover with a acrylic plastic polish made for the purpose. If there are scratches that need to be buffed out, you can use a very fine grade of jeweler's rouge.
There is a product called Plexol, that is made for aircraft windshields, motorcycle windshields, and anything made of clear plastic like plexiglas, lexan, acrylic, etc. It cleans, protects, and covers minor scratches. I use it on my motorcycle all the time. It is great stuff.