I have the schematic that one poster made on overclockers to create pot-adjustable PWM based on fixed 25KHz frequency using 556 timer IC but there is few short comings:

No low-impedance PWM drive for some fans that needs strong drive; I had ran into one of these before.
No option for inverted signal (easy with one extra transistor to invert the signal).

No multiple filterings to keep commutator noise out of the computer's power circuit, this is important thing when there sound card in the computer.
Lack of way to supply high current fans (this is my plan for specific use to create high pressure air project).
Need to drive number of fans individually of all types PWM and non-PWM fans by row of pots along one edge.
High current supply direct from power supply, up to 60W.

All on a one board shaped that can fit in a slot keep out space held by a metal bracket or mounted in a project box.

Include either PWM through inductor which I don't have skill to develop or another one that I do know how which is variable voltage regulators based on LT317 IC to vary the voltage for simple fans. Which I did create one LM317 with multi-turn pot back in the day to combat noise as turnkey solutions was not available back then this was for my socket 462 computer. I would love to use chopper through inductor for efficiency.

Do anyone had created similar to this and is on open source circuit board to create from?

Cheers,