You might find chicken wire a bit heavy and cumbersome if someone is going to wear it. Better I think to use pieces of stiff card, cut into honeycomb shapes and stuck together with masking tape to represent the pattern of the shell.

If you make it rigid enough before covering it with your papier mache it should hold together, just make sure that your adhesive is not too watery. It will help if you coat your cardboard "frame" with diluted pva glue. This will make it less absorbent, and so less likely to sag or distort. Cover the whole area with a layer of papier mache and leave to dry completely. You can then build up the layers to make a tough, lightweight turtle shell.