A hinged bag clip, printed as one assembly.
I tried printing http://www.thingiverse.com/thing:330151 once, but my printer didn't like the small gaps, so I wrote up a customizeable version of my own. They print nicely and work fine!
The arm will neatly rotate up to 90 degrees from the locked position.
Included are a 60mm, 80mm and a 120mm version STLs, you may choose your perfect length in the Customizer app if need be.
(Note, though, that the .scad wasn't made with perfect customization in mind; most of what you'd want to change is the length alone, but feel free to experiment!)
The arms should swing fairly freely after a bit of wiggling.