I have junk 4x4's on the dolly braces. I use 2 1" ratchet straps holding them down. I also put them on the frame braces and If I don't want them to be noticed or stolen I put them up under the bed between the frame rails. secured the same way. Out of sight since I don't use dunnage all that often.
Remember that any rough spots, cuts or abrasions on the straps used in securing the dunnage is the same as NOT having any securement at all if inspected.