when I made my last yarn bowl, I used my Foredom flex shaft rotary carving tool and started with a 1/8" drill bit. I drilled holes all along the perimeter of thee slot. Then I switched over to a carving burr and just slowly opened up the slot to the desired size. Be aware that if you get too aggressive with the drill bit or the carving burr that you can break the bowl. I don't have that experience but I know someone that did this very thing. They were able to glue the broken piece back in place with little to know sign of any damage. BTW, a Dremel tool works just as well.