120uH for 15 amps at only 20KHz does not sound right. The ripple will be heavy. I did a 1 amp Luxeon driver with a drum choke and found around 80KHz to be a better freq. Make your system with an adjustable freq so you can find the highest freq that keeps the transistor and coil reasonably cool.
You cannot expect to do a whole lot of filtering with a capacitor on the output, you need the inductor to do most of the work.
Frankly, for a fixed ratio, I would advise you to use a transformer. Size is not the issue- either a transformer or buck inductor can be large or small depending on the freq used. There are toroidal cores with 2:1 windings you could use, or maybe you could wind your own. As long as you have a core which works at high freq, you can do the job with small, low inductance devices.
The design issues are- DC resistance (not trivial with 15 amps!). There are many different types of cores with diff freq capabilities, if you exceed them, the core will heat up. If the core heats up due to wire resistance or too high a freq, the inductance and saturation current suffer. If you exceed the saturation current, the inductor partly stops being an inductor and ripple goes wild- basically, the inductor stops resisting the current change and acts as a wire, so the output will go to 24v for that part of the waveform.