you can build a buck converter and drive it from the output of a signal generator at whatever duty cycle you want. for your task, the 50 ohm output impedance is low enough to drive a pair of 50 amp mosfets.
the 1:1 coils from common mode chokes make good gate drivers for as low as 50khz into a mosfet, use a resistor in series with the gate for turn on, with a diode across it to turn it off quicker than it turns on, you get some dead time that way.
for a 250 watt buck converter you can use the ferrite transformer core from a computer power supply, gap it to about 1mm. maybe 10 turns of wire at 50khz. the capacitors on the input side of the buck converter matter a whole lot more than the output because the parasitic inductance between the mosfets and the capacitors causes significant problems.
anyhow you don't need a regulator, just set it to 12volts output which would be about 60% duty cycle. the voltage will drop under load a bit, but not enough to matter for your application. if the voltage drop is too high then get better mosfets and build a lower resistance inductor.
