I have done a small seashell mobile using a round wicker placemat as the frame. I hung the shells with sewing thread and then stiched the threads round the outside of the mat. I then stiched a couple of lengths of ribbon to the top of the mat in and X to hang it up from. I should have threaded a small ring onto the lengths of ribbon first. It would have made it much eaiser to hang. An embroidrey hoop would also be good to hang them from.
I drilled the shells bu puching them into modeling dough and using a drill on a very slow setting.
For DD I made a very simple one with felt birds. The frame was 2 sticks tied in a X shape with sewing thread. I hung a bird form each point and one in the center. It's OK though I do have to rebalence it from time to time.