Understood. I posted in that thread to see if I could get some further clarification. I didn't realize it was such a recent conversation. For some reason I thought it had happened in 2017 and I didn't want to necro post.
I'm going to be running a Samsung 960 Evo which is probably somewhat similar to the
950 Evo performance shown here. Under load, even a basic ad-hoc heat sink made out of a small 1.5mm thick aluminum sheet stuck to the SSD was able to quadruple the read time before throttling bottomed out performance . (40 seconds vs 160 seconds). And it bottomed out at a higher speed also. Note that this test was done on a back-mounted card though, which like you said, gets really poor airflow. On the graph, the purple line represents the bare SSD with a side-mounted fan blowing on it which performs well. I suspect in the S4M, being exposed to the ambient air due to the large ventilation holes but not directly being blown on by a fan would put the performance somewhere between the two.
Still, I'd like the option to have one if I decide to. Now it's a matter of finding one that would be able to fit.
There are plenty of cheap 3mm tall M.2 heat sinks out there for < $3
such as this one on ebay. It states it needs 5mm of clearance for the combination of thermal pad, heat sink, and bands that hold it on. I was thinking of using thermal paste instead to see if I could trim that profile down to fit it in the tight space between the mobo and the case wall.
I've also found more expensive ($9) copper heatsinks
such as this one which claims to be only 2mm tall, though it doesn't look as thin as the 3mm one above to me. If it really is 2mm, it might be a better fit in the S4M.
As for holding them on,
heat stabilized cable ties look appropriate for the task. According to this site, standard nylon zip ties are rated up to 185f/85C but are only good for a max sustained temp of 150F/65.5C. The monoprice page says they heat stabilized ones are rated up to 302F/150C. I know, I'm being overly cautious here. Normal cable ties would likely work fine for normal SSD operating temps.
The cable ties will add some thickness as well, so I might end up having to file grooves into the heat sink to countersink them.