I ultimately didn't do it, but my idea was to slant it slightly out at the bottom so it leans on the back wall. First, it'll be stable, second it allows light to reach the bottom of the wall. My other thought was, the moss would likely grow faster at the top, so if it's not attached, after a couple months I could flip the thing upside down, with the bottom now at the top.
Another idea I read online, great for tiny tanks, is use only 1 piece of mesh. The suggestion was to cut with a razor every other vertical piece of the mesh without cutting the edges. Then the moss can be woven, shoved in between these "slats" versus sandwiched.
Please post updates.