Can I change the shape of the clone tool nib?

From the Help guide...

To create a brush nib from an editable area

1. Define an editable area. (this means create a mask)
2. In the toolbox, click the Paint tool .
3. On the property bar, open the Brush category picker, and click a tool.
4. In the Brush settings docker, click the Nib options button on the Nib properties bar.
If the Brush settings docker is not open, click Window Dockers Brush settings.
5. Click Create from contents of a mask.
6. Type a value in the Nib size box.

Basically, create any image and put a mask around it. Then create the Nib from that,

Hope this helps