Here is a pack with pictures of walls and some fences, too, which you can use to cover things.
Mix and match pieces of walls.
Stick something on the sides to make it complete.
Once a wall is over a message, you can do a long tap on that message to reveal the text hidden below.
If you want to remove a sticker, just do the long tap, then tap "sticker details," then swipe left to reveal a delete button.