My favourite tool for sgraffito is a rubber tipped clay tool so I decided to try to fashion something along those lines. Here it is, my shiv if you will
It was made out of a plastic pickle fork, part of a water bottle cap and, of course, duct-tape. Worked quite well and gave me the control that I was looking for. Here is my drawing
Not my best but I had limited time today (but more than 5 min!!) and a lot of interruptions. I get to use my tool again tomorrow so we'll see what I come up with.