This article on Engadget gave me the idea - I already had a Microsoft Blue USB mouse that wasn't working anymore, a Zalman ZM-OP1 fan and the fan cage from my Shuttle SB61G2V3 XPC (I'm wasn't using the last two, as I've modded my Shuttle to mount a 92mm fan Vantec Stealth externally with an aluminium 80 to 92mm adapter).
Cut the mouse cable, strip out the red and black wires, those are the ones for power, we won't use the other two, used for transmitting data.
The Zalman OP-1 is a 12V DC fan, 0.14A, which makes it good for the job, as the USB ports output a 5V, 500mA current on each port. This fan is very quiet in normal operation, even quieter now that's running on 5V current. The fan has a 3pin connector, red and black wires are for power, and the third wire is for speed readings.
Mount the fan in the aluminium fan cage, add a 80mm silicone fan gasket and a fan grill to give it look a professional look ;-) and here is the finished "product".
This is quite useful if you use the R2H in it's stand, you can put the fan blowing air in the hard-disk air vent along the back of the unit. If you take the battery pack off you can get around 40 degrees C for HDD and 50 degrees C for CPU - not too bad.
I finally found the Thermaltake Mobilefan II at Performance-Pcs.com, and even if they weren't the cheapest around, it is one of the few websites shipping to Europe.