I made this for my truck a few years back with some angle/flat bar from home depot and some truck mud flaps from a auto parts store. I've since widened it by moving both sets of flaps out so there is now about a 6" space in the middle. I trailer everywhere I go and started to notice little marks in my hull on my old boat. Now with my dark blue/black hull I will never tow without it.
Rock tamers do the same thing. http://www.rocktamers.com/
I wanted more protection than they even provide. I also weighted them down so they don't flap in the wind as easy.