Equipment protection: How can I protect the scanner glass for exploration by scanning the depth of field and avoiding artifacts?

Recently I have got into the scan, but I have not yet found a good method to protect the scanner glass from objects that scratch or dirty it. I need to find a material that is optically neutral and thin.

I have tried acetate sheets (sold as document protectors), but they are only optically neutral for perfectly flat items, like documents. I have some transparent plastic document protectors, but they shrivel up and have undesirable reflective properties. I could use glass (I have a broken scanner), but unless the glass is incredibly thin, it will cut into my depth of field, which is important to me.

And that is The optimal material? It must be optically neutral, non-reflective, not prone to wrinkles, as thin as possible, and preferably inexpensive and / or reusable.