All fair points. It’s not finished yet.
Jobbo - I’ve still got 2x 2.5m lengths of tube left, I can put one right through the middle of the wheels and ratchet strap it down to the rack.
Rob - Front-back or side-to-side ? Or both ? The proper racks, which are welded of course, only have a relatively small gusset to brace the corners. I could replicate that with some plate & u-bolts.
Harry - that’s probably the main thing I was not sure about. Handrails are usually intended to be stationary, not bouncing along the motorway

The corner pieces are pretty hefty cast items though so I’m not sure a bolt right through would actually clamp it down onto the tube. Might need to shim it out to make it an interference fit ? Or cut a slot in the casting so it can clamp down ? Fill whole joint with epoxy ??