I've made bookshelf label holders from thin acetate sheet by cutting out a 7cm x 5cm piece, then lightly scoring and bending it to fit over a standard 19mm shelf.
It works okay-ish as a bookshelf label holder, but it's not "springy" like a real clip would be. Unfortunately, I have ~never~ found suitable transparent clips/holders, which is why I make my own from acetate sheet. (Okay, so there are these:
Previously, I used transparent glue dots to fix my holders' "flaps" to the over- and undersides of the shelf. These can't be seen unless one looks really closely. But the downside is that you have to painstakingly remove the glue dot residue from the shelf surface if you want to move a holder.
So how might I make the bent acetate sheet stiffer? I tried with a hot glue gun along the bend lines (after bending to a square profile), but you can see the glue. However, the principle works! The resultant holder is noticeably stiffer with enough "spring" to keep it in place on the shelf without glue dots.
Chemicals? Vinegar? Salt solution? Something to spray on the holders after bending, perhaps? Surely something must exist to make the plastic more brittle? Obviously the clear transparency of acetate needs to be preserved as much as possible.
Here is a picture of my holders, one in position on a pseudo bookshelf. The glue dots are visible in the pic, but not so much when you're in the room where the bookshelves are.
Note that I have 70+ categories, so that's a lot of label holders!
Cheers!
MM