Has anyone used them?

Yes, and it worked great!

Thank you, will try it.

What is it??

What a capital idea!! Or those very thin "similar substance" kitchen cutting mats I found once at WM or Target.

Jan in VA

I use plastic canvas in my totes and purses. Cheap and easy to use.

I'm imagining using an xray machine to stiffen fabric, which I'm sure is wrong... what are these?

I have been buying cheap plastic placemats at the dollar store and then cutting them to size. I cover them with matching fabric for the liner and they work great. I can get several from one placemat.

Be careful...if X-ray film gets wet it will stain anything, especially if it has been exposed(used)...found out the hard way when my fil was sorting film for recycling and ruined several of his good t-shirts,and the seat in his truck because he sat on a stack of wet film.

Not only will it stain the fabrics (because of silver solvent) but x-ray material is flammable.. had some x-rays in back of my car and the sun through the window scorched the films and the folder they were in. I do, however use them for templates.