>I can imagine a lot of heat dissipation for 50Ohm...
Not really. Look on the fan, it should have a label with the technical data. Small 12V 40mm fans are rated 1W or a bit above. Since you will be burning only a fraction of that in the resistor (50%?) and the rest in the fan, no problem with heat. Just get a 1W resistor for 10 cent.
