Any solution that will reduce the voltage from that 24 you sayd it is available to 21 means regulation, so that will not be a problem, but what is the problem is the current needed, 20A i by me too high for linear solutions cus overload and shorted output protection is mandatory at this current and the latter would imply insane amount of power dissipated on the series pass regulator. The obvious solution i think has to be a switching regulator, but even though it would cost less than linear one, still it will not be cheap, so the tradeoff will have to be the production cost, but not to worry it should not mean it would get one broke...
Now, what you have to provide us with is a test made on that power supply unit giving 24V ( or something like that ) with a resistive load of 20A, you have to make sure that the voltage will not drop under 24V at that current, as i understud it could be tweaked to give higher voltage, if possible than make it so, take is to about 25 or 26V and see how it handles the 20A current, only then can we know if a buck regulator can be possible ( it is the simples and cheapest i think ), otherwise some transformer solution will be needed.