I have not played with this set-up at all...just came to me this morning and out came the knife. I looked at the sag and came to the same conclusion...needs a support system to keep it in place. Someone else suggested a pickup cargo net to support the bag and that seems to make sense using some bungy to hold it up against the bottow of the hammock.
It was way too hot and humid to spend much time experimenting today but I think the concept is sound. Now I need to do some sewing and maybe wait for Fall to continue development.
Any other comments are welcome!!