Everyone knows that you can build one for yourself in an instant, but nobody can design one that will just work without too many component tweaks.
I substituted a BD437 for the BC140 (I suspect BC140 are hard to get these days.) And with no other changes, it runs a 8cm or a 12 centimetre fan perfectly well across their full operating range. (no heat sink on the BD437.)
Here is a pic of the circuit on breadboard - it should take very little time to compress this into a bit of vero or strip-board and upgrade it to a 2-channel controller.
I user a polyester cap for the .33uF. Not the trimmer on the pot must never be allowed to get to 2kOhms of Vcc else it will fry the discharge transistor behind pin2. My final board will have a 2k2Ohm in place.
Here, we are attached to a 12cm fan: