We put some truck size mud flaps on our Trackabout hire campers when they were taken to Camp York or Birdsville etc, they were held in with the large size zip ties and used two per hole, they lasted the entire trip, the bottoms of the flaps were half worn away from the constant pounding from the rocks but to my surprise the zip ties lasted. It will come down to the type of mud flaps you have on your 4x4, the bigger the better, This is not a great photo but it's the only one I had, you still get the idea,
Cheers
jk