I saw some plastic packaging online which would be perfect for my use case, but with it being plastic, I decided to print out of plastic to make a thing that doesn't use plastic. 1000 IQ play, I know. You can use it for bending cardstock fairly easily; place your card at the desired length, release the springs and push up. I use it for making keycap organiser holders when selling on eBay.

If there's demand for it, I may make an A5 and A4 version.

Required parts:
2x ⌀7mm diameter springs, 30mm length

1x cardstock_and_paper_folder_body

1x cardstock_and_paper_folder_bender