Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:3980079.

Small poster holder using rolling pin wedge principle, so we have gravity working for us. After printing wiggle the roller a bit so breaks free and is able to roll freely inside the housing. I used the tip of a ballpoint pen to nudge the roller free. For mounting I used some generic tack putty. Let me know if you want me to design a version with holes for nails or pins. Do not use supports! (also ignore the giant creases on my poster. It was used to test the viability of my design :P ) Update: added a version with bumps on the inside for those people who are having trouble getting the roller to grip properly.