Strictly speaking, these are not 120mm fans and the holes do not match up with the holes on the S5 mount, but they'll sit nicely inside the brace of the fan on an S5 (even S1 and S3), couple that with a slight mod (using this resistor: http://www.maplin.co.uk/p/wirewound-3-watt-33-ohm-resistor-w3r3 on the +ve line) plus double sided tape to hold them in position and you'll get a humming rig with more than enough CFM. If you put them in a pull configuration, the pressure will also help hold them in place. PS. All the mod does is reduce the voltage to ~7 volts (and thus cut its noise dramatically), so if you have a source with that voltage, use that!
You'll need to strip all the housing off and remain with just the fan.