A couple of Microwave-Oven-Transformers can give You
what ever Voltage You want to start with, and virtually unlimited Current, which should be regulated.
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If it is already working at 5V @20A = 100W
Then the resistance is 0.25Ω

A 1-ohm 100W rheostat in series will work.
Something like this:


https://www.amazon.com/Wirewound-Potentiometer-Variable-Resistor-Rheostat/dp/B07W7KVLYH/ref=sr_1_1

 

Unfortunately a Rheostat won't provide Automatic-Current-Regulation.
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Unfortunately a Rheostat won't provide Automatic-Current-Regulation.
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TS said he wanted to control the current. He didn’t say regulate.

 

I have another supply now that does that 5 VDC & 70 amps both constant and adjustable.

No you don’t.

Bob