Each Archivist needs two pieces of 3mm (1/8″), annealed plate glass with a 50 degree miter on one long edge. The long edge is 16 3/8″, the short edge 12 inches. If you’re doing this in a metric country, +/-1mm should be OK.
This glass is sometimes called double strength glass, and it’s also called 3 millimeter glass referring to the thickness. On one long side there should be a mitered edge — a glass working term that means angled. I have used glass mitered at 45 degrees and shipped with each kit glass mitered at the ideal angle which is 50 degrees.
If it’s difficult or impossible to get glass like this, you get one long edge sanded into what is called a pencil edge.