If I where to do that, I would:
1: Set up a current limiter and a regular opamp voltage follower to ensure a stable output voltage.
2: Design a Buck-boost converter, include an oscillator circuit with an input that regulates the oscillator duty cycle output.
3: Melt the two designs together.
4: Ensure that the design can withstand at least 240*1.5*sqrt(2) volts (1.5 for least safety, but that you can increase)