From Tekkotsu Wiki
Planar markers can be attached to walls to help a robot navigate around an arena or maze. See the discussion of Cylindrical Markers for basic information on how navigation markers work. The simplest planar markers are bicolor markers, which work the same way as bicolor cylindrical markers. To the robot, they're identical.
Where to get supplies
Planar markers can be constructed from a stiff backing (posterboard or similar material) and colored construction paper, with an optional velcro tab on the back.
See the discussion of Cylindrical Markers for advice on construction paper.
For velcro tabs, go to [McMaster-Carr] and purchase 2 inch wide, adhesive-backed nylon hook material (item # 9489K544) and adhesive-backed nylon loop material (item # 9489K878).
Cut the plasticor into 4.25 inch squares. Cut the construction paper into 2"x4" strips. To make bicolor markers, use a glue stick to affix two strips of construction paper to each square, leaving a 1/4" gap between them.
If using velcro, cut 1/2 inch strips of hook and loop and affix one piece to the back of the plasticor and the matching piece to the wall where you want the landmark to go.
See the discussion of Cylindrical Markers for instructions on testing your markers.