So, I have this 12V 40x40mm server fan. It's *loud* but it does have a wire that accepts a PWM signal.
Not only is this whole COVID-19 thing going on, but it's a public holiday so no chance of help from the outside. Looking around in some rubbish I've laying around, I pried loose what I could, and stuck it together.
No ordering any PCBs for this, no buying any parts. Just some snips, and a soldering iron.