The HUG was designed as part of a system to take advantage of an under quilt. I suppose adding a few more shock cord to strap the back up.
OR, you could build a HUG that is more like a half tube net, e.g., sewn in the back. The netting would need to be longer in order to do this.
Again, the HUG was designed to be minimal and take advantage of other systems. Without those systems in place you're going to need to get creative.