Three letters..........PVC.
I cut a nice 3" round hole in my case with a hole cutting bit from home depot. Bought a piece of what I think is black PVC.... which had one end 3" and it tappered to a larger end. I cut a section of the 3" side and used epoxy to glue it to the hole in the side of my case. The inside of the pipe lined up exactly with the hole in the side of my case. The hole is also the same size hole in an 80mm fan. I used that setup for a while. It lowered my cpu temps around 5C. The same cpu temp I would get if the side of the case wasn't on. I covered the hole with a 80mm guard, and a metal mesh filter. It looked pretty nice.
Since then I have taken the plain duct off. I broke the bond of the epoxy and the duct came right off with the epoxy leaving no mess on the inside of my case. I added a smart fan 2 in place of the duct. Then added a square duct around the smart fan which feeds my Tornado fan on my cpu nice cool air. The square duct is not totally square its missing the front side, and the bottom side or the lower right hand corner. I tested the square all different ways and this lowered my cpu temps the most. This is even better then a closed square which wasn't all that great. Having 2 sides open in the front and bottom direction helps to also send the coolr air towards the Northbridge, and the memory on my NF7-S. Just make a nice square duct out of whatever, fit it over a fan....and cut off some of the sides if you want. I made the duct out of nice blank cardboard. Glued together with Elmers.
Since then I have added a thermal probe glued to the side of my SlK-800 with themal epoxy. I modified it by adding 2 50k resistors in parrallel (Total 25k ohms) with the probe to make the probe more aggressive. It ended up being slightly too aggressive. So I then used the pot that comes with the smart fan 2 and soldered that in series on one side of the probe. I use the pot to adjust what I want the lower temp for the smart fan to be. Then the probe does the rest.
So keep this in mind, you don't need to put the thermal sensor under the heatsink. You can put it on the side and just make it run more agressive. Keeps you from having to take the heatsink off. I found even under the heatsink the probe wasn't agressive enough.
