If you want to make a bowl from something so irregularly shaped, there are two different things that you could do to make the whole process safer. There are various epoxy resins which have excellent adhesive (and filling) properties. If you use this kind of material, you could establish this block of wood as a "solidified" piece, which is in far less danger of flying apart. The other thing I would suggest is that you could use a "router style" cutting blade in a small angle grinder, to preshape your work piece. Rather than having a rapidly rotating (out of balance) piece of wood moving past a stationary chisel, you could then have a rapidly moving cutting blade held in a fixed position against a slowly rotating irregular piece of wood.