A reversible quick connector tho hold bags or other stuff. The hooks snap in very pleasently and can take quite a bit of force. You may use it in conjunction with: http://www.thingiverse.com/thing:45219

Keep the U hooks long and thinn enough to flex. Print two U-Hooks one Snap-in-Unit and two Lock-Blocks. The lock blocks should stay in by friction. If they don't use adhesive tape to hold them in.

Use "customizer" for deviating sizes.