Multione supply a steering lock bar but I've never used it.
I'm a big fan of the 38mm straps. Still a good breaking capacity but easier to handle than the 50mm ones. I have six of them cut down and sealed stashed on every trailer. Strap it so it's pulling fore and aft (or to the centre on certain awkward things) and it's as safe as it can be. I find that a lot easier than throwing a long strap over, walking round the other side of the trailer and then praying it doesn't rub through on the journey.
If you're just strapping something like an articulated loader straight down to the sides, then yes, it's likely to move. Strap it so it's all under tension. If a machine or trailer doesn't have perfect lashing points, I weld them on. Life's too short to struggle.
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