I know I know, buck converters are a dime a dozen, but I really needed something in a pinch so I made this board without researching existing options.
After some research, however, I found that most solutions to this problem involve inefficient linear regulators, some DIY buck converter using resistors (voltage divider), or el cheapo buck converters with questionable parts.
Here's one example: https://www.antonmansson.com/how-to-run-12v-fan-on-24v-3d-printer/
OK so that solution is inexpensive, at ~$2 per module, however it's a pain requiring one module per fan, and having to tune each one manually for each fan (requires a multimeter).
My idea uses a simple dual converter that connects to a PC fan controller which is designed specifically for controlling fans with turn knobs (and it looks cool).
So, I'm thinking of designing a tiny plug-n-play PCB to make this simpler for others. Let me know if that interests you.