I actually went back after I took that picture and slid a little heatshrink over the resistor/wire to insulate it better.  I'd highly recommend doing this if you go that route as it will prevent any accidental shorts against the inside heatsink.  Also, just for reference the resistors I used were mostly 39 Ohms.  I may have had a couple 45Ohm in the mix, but it lowers the fan speed enough that it still blows a good amount of air but isn't as annoyingly loud as the straight 12v.