Greetings,
Just one potential direction you might go in is to separate the straps from the back and have them be two items completely makehuman side. Then follow exactly what you did using the (*) category to create the solid/rigid asset on the bag alone. But for the straps make them as you would for instance a bathing suit top and skin them properly to the model. I have had success with this in past and in fact often break items into multiple pieces for proper effect mh side and for porting. There are many things that may work for you here, but thats my suggestion off top of my head,
Elv