A hot wire knife is a great way to get the rough shape without making a mess. The flip side is that edges cut that way are very smooth, not like something found in the natural world. Once you are close you can rough up the surface many ways. I have recently used a combination of 60 grit sandpaper and dragging a pumice stone across the surface and really liked the final effect.
If you search for a thread called Halloween 2017 you will see what I did.
Have fun. FYI hot knives are readily available at craft stores like Joanne Fabrics or Michael’s.
