I missed this thread earlier, somewhere I have some spare glass. The problem is the thickness as it can no longer be sold direct to the public, 2mm is the minimum thickness. I still have a small supply of 1.5mm gl;ass which is ideal for these neg carriers.
My preference with Dursts is to use glass on the bottom and the metal frame in the top. I made my own 35mm masks back in the 1970's and still use them today. It was very simple to do using thin aluminium.