I have not had a struggle with Newton's rings, and also use a 1/4" piece of plate glass from the local shop. I use this for proofs and for contact sheets.

Laminated glass with eased edges would be hard to break and should do a good job, also.

If I ever get Newton's rings, I will pursue the non-glare glass solution. I bet this is available in a laminated sheet.

As Sal mentioned, be sure to tell the shop what you are using it for so they select a section without scratches or other defects.