[footfootfoot]

Steve, I don't know how big that thing is, but my full face respirator has these tear away plastic guards which prevent the face shield from scratching. There is a stack of three of them that adhere to the shield, when they get boogered up you peel it off and so on until you've none left.

Motocross riders use them too for mud. The shields are usually polycarbonate so the adhesive isn't an issue.

Another trick which may work is to use acetate sheets available at most drafting supply or art supply places and carefully cut them to size and with a drop or two of glycerin squeegee the acetate in place. Not sure how well this will work, but we used to do that to get color correction gels to stick to windows for shoots.

Clear contact paper?