At the moment I'm thinking getting a mandrel made from aluminium tube; take it to metal workers and get them to bend me a 120 degree circle.  My concern is would I be able to break the part from the mandrel even if it was a perfect curve without being able to apply any torque to it.  Would I need to step down a size and apply a thin wall silicon tube to it to break away cleanly and handle any deviations in the curve?