After reading the rave review of bone folders in Kevin Kelly's Cool Tools https://kk.org/cooltools/bone-folder/ my immediate thought was, "I can print that."

A quick search found Bone Folders by cantloupe https://www.thingiverse.com/thing:2024795 but they looked impossible to print.

So, I made a trivial modification, and give it back to you.

This flat bottom version is easy to print and works great. It also makes a nice letter opener.