The plastic mounts have a threaded pipe that they clamp on to. By the time you move the fender out far enough to clear the clamp, the fender will be sticking out well past the edge of the trailer.....like 3-4".

Here's pictures of a local trailer we copied the mounting on below, but remember, this fender is only 20" wide to cover a Super Single. When you get fenders wide enough to cover duals is when you get into trouble. I'm assuming you have a Neway suspension.

This is looking at the front of the front fender mount. Notice in these pictures the 1/2" flat iron was cut off of the threaded pipes and pipes welded directly on.

This is looking between the tandem fenders. No welding to the frame here either. Only to the plate welded to the frame, and to the channel iron underneath.

This is looking up under the back of the trailer. This one is welded to the main tube about 4" from the very back for lack of a place to mount. Poor picture I know but it was raining that day.

We painted ours before it left the shop to prevent the rusting you see above. I highly recommend it. :)