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It sounds like you want this:
4.20mm Pitch Mini-Fit® BMI Plug Housing + crimp terminals
to this:
2.5mmID x 5.5mmOD, +12V center
Just a guess...I'm thinking you need ~80+ watts (10A spec), which will require all six pins of the 75 watt PCIe power connector.
Note: Pins 1 & 3 of PCIe power are 12V @ 8A max physical; note that in the PCI-SIG spec, Pin 2 is not connected, and pin5 is ground-sensing (i.e. required but minimal current TO a ground). Probably most power supplies are dumb and wire like the colors show, but you need to follow the specs making a cable or you will find the exception.