A different build but it may help: I built an insert for my utility trailer, it sets on top of the trailer rail and extends over the fenders. Center of trailer to center of first 2X4 bunk rail = 17 inches, center of first bunk rail to center of second = 18 inches. Rode great with 2 skiis but for just one I added a 5th bunk rail 18 inches on center from one of the inside rails so a single will set closer to the middle.