I left the original side tabs on the box in place and reinforced them with clear tape. The doorway is cut 1 inch wide, 3 inches high. This makes it fit really snuggly over the wooden tracks but I thought that was a good thing because it’ll fall down less.
I asked my son to cut rectangles out of a rag dish towel and I glued them into the box with a hot glue gun. I poked a hole through the top with a nail and glued a nut there. The barrel will pour the water through the hole and the rags scrub the engines as they go through the washdown shed.