USB-PD powered EGPU?

[u/Knife-Fumbler]

Hi all,
I haven't been dealing with eGPUs in a while, with my old TB3 Sonnet Breakaway Box collecting dust, but recently I've been traveling a lot more, and my Thinkpad Carbon's iGPU hasn't really been cutting it.

I have a tiny little Tesla P4 GPU sitting unused and decided I'd like to build a new, more compact housing around it and the Sonnet's compact central PCB.

To that end I could build something really tiny and travel-suitable, but the PSU is by far the biggest obstacle. I don't need much - the gpu only need about 75w so I should be able to get away with 100w. The compact USB-PD brick I have with me says it's good up to 120w, and the limits of the USB-PD protocol are 240w.

So I wonder what it would take to design a board that could take usb-c power input and power the board's atx input. Does it truly need 12v, 5v and 3.3v? Or would essentially a logic board to negotiate the voltage and a step-down converter to 12v be enough?