Guys, I found that the heatsink is not adhesive. It was not an issue for me since I have some silicone thermal glue, but I figured many users risk not to install it or to poorly fix it, then to report stability issues.
That made me think that I've had very good success using these adhesive heatsinks with other RK3288 devices, they handled the load pretty well without any fan, even when overclocked to 2 GHz :
Here it seems there's a little more room on the board and that a 40x40 will fit as well with even more efficiency, possibly even a 12mm high one but I didn't find them. Here's the same in 40x40x10 with an adhesive pad :
That should allow to completely stop the fan for most workloads, even when installed in the metal case. Unfortunately I already installed the small one so I'm not sure if I'll be able to remove it and replace it with the larger one.
I just thought you could be interested.