This is an attachment for the Root Robot that allows the robots to paint wide stripes like a zamboni. This attachment fits into the pen holder on the robot, but notice the up-down is reversed.
More information about the Robot and programming it can be found at: https://rootrobotics.com/
I folded a paper towel into the trailer, and coated it in ink paint. Felt or other fabric would also work. The large pocket on the top center is to counter balance the weight with quarters. 6 x bolts (M2 or equivalently small), washers and nuts are needed, as well as 0.25" magnets.