They are acrylic, not too stiff and best of all inexpensive. I found them at the chain store AC Moore for $4.99 and used a 40% off coupon. (They only accept one at a time)
When I spotted them the first thing I thought of was storage for my "fabrics in waiting". I purchase fabrics with coupons I like and hold them until I have all the fabrics I want for the quilt project. Stacked in my double ottoman sorted by project. With this method they can get dusty. One of the boxes I will be using for extra cut fabric squares from my green quilt. I have made this one 3x's its popular.
They are 12x12 so my layer cakes fit perfectly plus a couple strip packs. They also have bubbles in each corner so the lock together when stacked
I love a good find, when I am in the area again I will get another -print those coupons!
It has taken up to a year to find the perfect fabrics with the perfect pattern. Does it mean I sometimes have extra fabric after a project? Absolutely, that's what scrappy quilts are for, and zipper bags, and purses and applique and and and :)