No you definitely do NOT want to do a triangular cannel. Bowl gouges generally have a fairly deep "U" shaped cannel.Well what I thought of using was either an old tie rod or anti sway bar off a junked car. Those should be a high carbon steel. If necessary I could turn the diameter down on a metal lathe.
Having never used a bowl gouge should you try and work a triangular flute or a more rounded bottom? This would be done with a hand held grinder with either a grinding wheel or a cutting disc. Of course I'd have to go through hardening and annealing it afterwords.
The worst it can do is not work correct?