Paul TBO: This is a pic of the String Swing ($10 from Musician's Friend).
The plate screws to your head (or whatever is high enough on your stack). Its flat, so I've never had any problems when hauling gear. The hook screws into the plate at the gig, and your bass hangs there.
I used the side away from the edge of the stage, so my bass is protected from the great unwashed on breaks. NB: my amp head is about 37 lbs. and low profile, so the bass has NO chance of tipping. If you attach to a rack, consider the lowest mounting point that will serve your need, so that physics and gravity don't make this a "learning experience". I imagine you could hang basses from each side, and maybe 2 on the back if you'd like! Larry (The Wide One)
