Given my experience of trailering Teela's bike and an ST1300 over thousands of kilometres I wouldn't put my valuable bike on something so flimsy
The rough roads around Oz will soon bring that little trailer undone
You need a minimum of an 8x5 trailer to easily take the size of an ST. I have worked out an effective tie down system by using the tip over rails and tying them down to the front outside corners of the trailer. Then one tie down strap looped over the rear wheel with out panniers on, to hold the rear of the bike securely in the centre of the trailer.
There is no need for handlebar straps as pushing your weight down onto the handlebars as you tighten the tie downs holds the ST inplace nicely. I also have added a neat stone guard onto my trailer to stop any wayward stones. I have also had a wide & longer ramp made to make loading easy.
My gal trailer was sourced for $800 and a few mods, & led lights brought the cost up $1000. A cheap investment for that added peace of mind knowing your bike will arrive safe and sound if you ever need to trailer it.