Here's a photo of a "prayer wall" I saw at a church I visited. Folks could write out prayers and tuck them into a "stone." This was constructed using various stone-colored felt. Fake plants were tucked into the "stones" for effect.
The idea is based on the popular practice of praying at "The Wailing Wall" in Jerusalem. Jews and Christians will write and tuck prayers into the cracks between the stones of the wall that is considered to be remnants of the western wall of the Temple's original foundation, in effect, "posting" your prayer close to where ancient followers believed God dwelled in a special way.