Interested in this topic as well. Only one m.2 on rear - ASrock x570i phantom. Any other rear M.2 heat issues or fixes would appreciate, not sure if riser cable adjustment is the way to go. Compatible heatsinks info also appreciated.
I really don't see any downsides to routing the riser cable behind the GPU.
Once you've rerouted the cable, there's only 2-3mm of clearance between the surface of the M.2 card and the motherboard plate. This isn't enough room for an additional radiative heatsink (i.e., a piece of metal with fins); and though you can find M.2-size copper plates, I'm skeptical one would help. (You still have to transfer the heat to the copper, and from the copper to elsewhere.) Even if you could get a radiative heatsink back there, you'd need airflow between the motherboard and the mounting plate to make it effective.
Let's don't forget about thickness of thermal pad between ssd and radator (about 1mm). So roughly said the radiator must be 1mm too
I think the idea here is to use the motherboard mounting plate as the radiator. A thermal pad just needs to bridge the gap between the M.2 card surface and the plate.
These M.2 are designed to work in hot invoriment like Ultrabooks.
FWIW, it's not unusual to find a thermal pad between an M.2 card and the metal case of a laptop (when the laptop has a metal case).
2mm space for a heatsink on backside of motherboard
Hm... I thought I measured 3mm. Welp, we'll see if the
pad I ordered fits.
Ouch, wrong pad, not the Silverstone, but this one:
Cooling Junkies Wärmeleitpad Thermal Pad Industriequalität 120x20x3mm 7W/mK Wärmeleitfähigkeit
German text, because I bought it on Amazon.de.
I did not use the Silverstone pad yet.
Ah, that's reassuring; that looks very similar to the pad I've ordered.
The Silverstone kit looks like it includes two pads: a 0.5mm and a 1.5mm. I don't think those are thick enough to bridge the gap; though they might make it if you stacked them. (Also, the thermal conductivity rating for these pads is just okay, not great. You can probably find better, cheaper generic pads without the Silverstone branding.)