The pc does not have back compartment, nor anyway to attack the cable to. What I am thinking right now is to hot glue some zip ties to the centre gap between HDD and mb. And then zip ties the cable to the zip tie, so it runs down the centre of the pc cleanly and branches out to its destination like a upside-down tree.
To get anywhere I think you’d need an extension for the 4 pin CPU power cable. That one going diagonal is the worst thing here and zip ties can’t fix it.