Here's a simple DIY fan switch. I've seen this technique used in SPCR forums before, so it is in no way an original idea. Given the number of questions about fan controllers, I thought it might be useful for others to see how easy it is to make their own. The whole thing cost me about $5 and took half an hour to make, even with my crude soldering skills. The 12-position switch provides voltages from 12V down to about 3.75V, which should cover most requirements. The diodes used are 1N4004 which are easily available and are rated at 1A, so there is no fear of overloading anything. I think any diode from the 1N400x series would be fine. Positive 12V is applied to the silver wire at the bottom, and the positive terminal of the fan is connected to the red wire which has been soldered to the pole of the switch. Note that the diodes are all facing in the same direction, with positive applied to the anode, which is the end without the silver band on it (important!).
Each diode drops the voltage by about 0.75V, so you can make up whatever combinations you like, eg a simple toggle switch with 6V and 4.5V positions, etc.
If you're just looking for a safe way of getting just under 7V, you can simply join seven diodes in series without the switch, or 6 diodes will give you about 7.5V. The diodes only cost about 5 or 10 cents each.