There are some very expensive aerospace units available. I read about them in a tech magazine around a year ago. They are pushing the 100 watt per cubic inch mark! But at around $10 plus a watt to do it!
For that wide or working voltage range you specified and at theat amps output I would consider a rework job of a higher wattage computer power supply. They are the smallest and highest wattage multi voltage output units I can think of that are easy to find and fair to adapt to custom applications. They can, with a little rework of the voltage regulating circuits, easily give you 3 to 15 volt outputs all with 20 amps plus current capacity. And putting a pair of digital meters in them is fairly simple.
It does take some electronics know how and a little time but I have reworked several computer power supplies to give non stock voltage and current outputs and have had them work reliably afterword.
Its just one option to think about.