So, before u-bolting the aluminum square bar (from homedepot), I wrap the tie down around the trailer 's tie down loop because there is not enough room to use the hook itself. If you are only doing 2 bikes, I would recommend u bolting the center tie down (renforce the bar better, and the chocks won't move forward).
If you needed a center chock, I pre wrap 2 tie downs around the trailer 's loop and around the bar, lay the center chock down, then tighten down the 2 bikes, it secures the center point.
Before this trailer i trusted just having the chock and 2 tie downs to secure the bikes, but since there are sliders and walls evolved, have 2 straps on the rear ends to prevent side to side motion (my bikes use the middle tie downs not the very back ones) the blue bike closest to the slider only has 1/4-1/2" gap to the slider, never touches
Also note, I used a second tie down on the clutch side of the blue bike to bring the tie down back so it doesn't rub on the slider (don't need the slider cutting it)