You could use a simple method such as burning off the excess voltage,
but as you can see, at 16Vin, 5v@5a out, you need to dissipate 55watts
of power. Not a good scenario. Your overall system efficiency will be 31%.
Save yourself time and money, and buy a point of load converter.
The synchronous buck topology is simple and very efficient. The entire supply will be about the size of a cigarette lighter. (volumetrically).
Plus, these run at typical efficiencies(%) in the low to mid 90s!
Look at Wall Industries, Datel, Tyco....just to name a few.
S.P.