But those two are not really what you are probably looking for.
I think you last and least janky solution outside of designing your own PCB would be a PWM to USB adapter.
Besides that, I saw some fan Hubs powered by Molex. If you have any sata or molex power in thr back of the server it might fit there and look more proper.
Remember though: If the airflow of the server fans are too great on the fan it could turn the fan and induce an electric energy into the circuit.