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

Another idea would be to "flatten" the brush on the Brush properties docker. Use the blue handle on the brush diagram or change the number beside Flatten. The other blue handle on the left rotates the brush. You can save the brush to use again later.

Patti