I just removed the internals of a dead IIfx power supply and got a random server psu from ebay to put inside the original shell.
I drilled out the original standoffs and drilled new holes to add standoffs to match the hole pattern on the replacement.
I didn’t bother putting a solution for the passthrough power connector back in place. In the guide you posted the passthrough will always be powered once the Mac is plugged in. The original power supply will switch the passthrough off when the Mac is powered down.
That’s as easy as it gets without having to do 3D printed mounts and panels and all that stuff.