I am in the process of changing the PolyBot design from using these: http://www.thingiverse.com/thing:4962

To using these: http://www.thingiverse.com/thing:5126

During that process, I find that I need a simple shelf type of thing that's in the same theme as the ball and socket joints.

This thing is a rod mountable socket. That is, one end can be slipped over a rod (5/16 by default), and the other end has a single socket that is appropriate for that ball and socket connection system.

Of course, it's a building block, so you can use them to do anything you want from building simple shelves, to making cross support for trestle bridges.

The one in the .stl file fits 5/16" rod with a slip fit (it's loose). I generally put a wrapping of tape around the place I want it to stick, and friction fit it over the tape. Alternatively, you could just glue it into place.

If you are using threaded rod, then you can change the OpenScad to use a 1.05 factor on the inner radius, and then 'screw' the fitting onto the fitted rod for a fairly snug connection.

1) Look at your framing design for places where you might be able to use such a thing.
2) Print as many as you need
3) Print out ball caps for your 5/16" rods
4) Place ball caps on ends of rods
5) Slip the sockets onto the rods where needed
6) Press fit the capped rods into sockets
7) Rejoice!