Hello,
Just finished modding my new Pi3. Before modding I gave it a stress test for 20 mins using Sysbench prime calucation. I was getting 78~81C on ARM CPU. So I decided to do something about it before the hot summer arrives.
The codes I used for the stress test... you can download sysbench by typying "sudo apt-get install sysbench"
I dug out some Pentium II heat sink and started hacking away using dremel. I tried to mod as large piece of heatsink as possible in an slim aftermarket Pi case; I had to sand for 2 hours using 180 grid sand paper to get the heatsink down to perfect size.
I used Laird thermal pad (those used on GTX 980Ti GDDR5 rams) for TIM between heatsink and CPU. Larid thermal pads are petty expensive little sucker at 1USD a pop...since they have 5W/m.k. thermal conductivity it's all good. Also used some electrical tape for protecting those taller IC components around the chips. Note that I didn't use thermal paste because I want to create more distance between the heatsink and IC components on board.
I slapped on the heatsink on using a little bit of glue gun make to make sure heat sink is firmly installed on the board. (Press down hard while the glue cures). I used glue gun because it makes it easier to remove in the future.
I dremel a perfect round hole on top of the case for a 12V 4cm Fan, which I will be running at 5V. This makes the fan so quiet I have to stick my ear to it to hear it. Put some shinny fan grill to make it good look 
For the DDR2 ram, I used Silicon cooling pad to spread the heat and transfer some heat the case (every little bit helps!). The pad was too thick so I sliced it in half, but still thick enough to touch the bottom of the case with some pressure.
Putting everything together and run the same stress test for 30 mins~~~
Check out those sexy exhaust ports in the back~
The result I got was 49~51C!!! Bang! feels good for three hours of work!
