If you don't have anything good to say then be quite and go to your room!
Ok.
@OP,
If it is not too much trouble, would it be at all possible to find out what "the device" is ? (FYI, that is what the Manhatten Project scientist called the bomb when they discussed it , "the device". What a coicidence)
If the load current is 0.3A , and the PS voltage is 24V, then you can use a dropping resistor to drop the voltage ,
Dropping Resistor Formula
Rdrop=Vin-Vload/Iload= (24V-12V)/0.3A=40 ohms==>RDropping=40 ohms.
Power Dissipated by Dropping Resistor
PR=I X V = 12v x 0.3A = 3.6W (Not a standard value. Next highest standard value is 5W)