I would use thinner wood for the mules and cut them so the angle is thinner on top of the mule. OR i would cut that board in 1/2, do angled cuts so the mules can be 'pushed' through. Reattach with glue or attach to a backer board to join them. OR make the mules of thinner wood, maybe 1/4 or 1/3 inch and round the edges with a flap sander in your drill press. I would stain the back board with a light stain and the mules with dark stain for contrast. I look forward to seeing the finishe project.