Because of their size, large fenders may have a tendency to move around in strong winds.  If this is causing you a problem, put some water inside the fender to weigh it down. Before you inflate the fender, lock the valve open and put a couple of litres of fresh water inside the fender with a hose, unlock the valve then blow it up like normal. The water will sit in the bottom of the fender and stop it from moving around so much.  When doing this, be sure to have the valve located at the top of the fender so the water doesn't sit against the valve.