Just because I can't seem to stop tinkering with this bag, I found these online:
I'm going to be picking up a set in the near future and see if I can't make an easy clip-on backpack conversion from the sling style. Extra loops? Sternum strap? Sign me up. Could open a lot of opportunities for DIY bags out there...
And they're built to carry guitar cases and sax cases that weigh more than our bags do, so I'm guessing they're pretty sturdy and comfortable.