Two years ago, I began a project where I was to scan some 12k in negatives (the project fell apart after 3k due to mis-communication). Having completed 3k.

Some of the negatives were 4 x 5's and others were 3 x 4's.
For the 3 x 4's I consulted a friend whom owns a machine shop. He showed me some machinist plastic that is sold in 8x10 sheets (their own use was using the thicker plastic to scrape the liquids off objects on an assembly line), with the minimum thickness being 0.0008 (8/ten-thousands) of an inch.
He sent the plastic out to another supplier who cut the plastic using an oil-coiled router, thus preventing the plastic from melting while cutting.
The process wasn't cheap even though my friends did his share for free.
I did end up with extra templates (the 3x4's were laid into the 4x5 holders) and worked just fine.

Initially I tried using thin matting, however they lost their firmness after a few uses.