The width of the 4 wood "posts" on the earthen bumper ARE too wide for the flush end of the car. If you glued a small wood square to the front of the 4 posts, it would work perfectly... speaking from experience.
The square need to be about 1/2" x 1/2" and glued so the the flush area at the top of the handcar clears it, as well as the two side flush areas. A round rubber bumper works well too and "cushions the blow".
Also, put a screw in the earthen bumper, or it may bump out of position.