OK, here is an idea. The screws that protrude from the walls into your space likely extend our 1/2" or more from the wall.
Cut some square blocks from 2/4 stock. Drill a hole into the middle of each block that is the screw size. Fill each hole with epoxy-flox mix. Press the block over the screw and allow to cure. You may need to use tape to hold the blocks in place during the cure.
With blocks in place, attach a panel to those blocks that your posters can be attached to.
Best of luck,
Wes
N78PS