1 Dell PC fan (junk)
1 LM2596 adjustable step down DC-DC converter (ebay, $1.54 shipped)
1 10K ohm linear taper potentiometer + knob (Tayda, $0.50 + $0.22)
5pcs Neodymium disc magnets, 3mm x 21mm (ebay, $1.35 shipped)
1 large penny washer (Ace hardware, $0.37)
1 barrel-type DC power jack 2.1mm (Tayda, $0.16)
1 12V power supply (junk)
1 food container
3-pin angle header, 0.1" pitch (Tayda, from 40 pin strip $0.17) (optional)
3 female-female jumpers (ebay, from 40-way strip, $2.80 shipped) (optional)
The step down DC converter came with a 10K multiturn trimmer that was too fiddly to use, so desoldered it and put a 3-pin header in its place. I connected a 10K potentiometer to the header using jumpers, but I could have just soldered wires in place. All that remained was to solder the barrel jack to the inputs and the fan to the outputs, and hot glue the fan in place. I used epoxy glue to stick a penny washer on the top of the fan and stick the Neodymium disc magnets to the washer. Total cost, less than $5 but I had a few bits and pieces lying around.
If I can be bothered, I'll put in a little microcontroller like a Attiny2313 to monitor the tachometer output from the fan and show the RPM on a 4 digit LED display.
Edit: RPM meter up and running but needs a few tweaks.