What suggestions would you have regarding making one's own dark slide film holder for large format-----specifically the material, where, common or not so common?????

The dark slide (black) in 2 of my oversized film holders are well worn on the corners--they are original Rochester film holders. I would like to replace them with something perhaps modern--plastic inserts which could be cut to specific size--and thickness of course an issue

Has anyone made their own dark slides-----your experience please

Thank you