Nice looking job. One suggestion I have is that you make the front post attachment a little stronger. As it is, it doesn't have much strength if the boat is on the tracks and it moves forward or backwards when the front post in on the ground. Something will bend and break pretty quickly. Maybe you could replace the straps on each side of the post that attach to the main bar with triangle shaped plates that have two attachment pins into the main bar. Or maybe just add an angle brace?
Jack