Shug's solution is to run a shock cord clipped from one side, over the ridge line, d clipped again back to the other side of the UQ. That certainly takes care of the UQ falling off the body. So does clipping to the side.
The original suspension running shock cord through a channel to both suspend and pull the side flush was not optimal. The heavier the UQ, the more obvious the geometric and physical problems were.
To bring the sides of the UQ in, the same light shock cord (or even lighter elastic) that is typical of the end channels and commonplace in garments might be wanted, but it should end at the corners of the UQ.
Windows cut in the side channel can provide access to that cord or elastic, and they could line up with and hang from hooks attached to the side of the hammock.