Hello David, thank you for confirming white painted portions identifying water can.
Jakko, thank you for the continued interest. A flat metal strip (same as those used for the constructed rack) with narrow ends is what I had originally thought. Taking a close up view of the area, it does seem the 'restraining' metal strap is just visible to the left of the rack framework:
If one of the narrow ends is curved, it would be very slight bend as there is not much room to pivot at that point in between the fuel can handles. Also the actual insertion of that strap would require prying outwards one end of the rack, so not sure how viable that is?
Another example, but this one is designed differently with the vertical framework tapered to a point where the restraint is inserted:
