Well, it is a bit early for that I guess. I don't even know if it will work out yet.
What I do know is how my fanless Mini PC is working like:
That system has a 28W APU (7640U). With two flat heat pipes and those heat pipes have not too efficient contact with the heat sink via 1 mm thermal pads. That is good enough. There is thermothrottling during full single core load and its hitting thermal limit during lasting full multicore load. Interestingly with full iGPU load during games temperatures stay pretty decent at less than 70°C (even though RAM is hitting thermal limit). From that system I also know that the heatsink can manage to reach 55-60°C or so (just a bit too hot to touch for longer than a few seconds).
The Mini-PC heat sink is actually a LED heatsink (PADLED-13080) and has a technical data sheet:
https://wakefieldthermal.com/content/data_sheets/PADLED_WT_v2.pdf
At a deltaT to the room temperature of app. 30°C, the data sheet would specify a power dissipation of 33W. Close enough I think to what I see in reality. The data sheet is listing a thermal resistance of 1.5 °C/W. But based on the actual numbers it would look more like 1.0 °C/W. Anyhow, it is a starting point.
I found extruded heatsinks with technical data sheets and took the profile of one as inspiration for my heat sink. The commercial one would have a width of 200 mm and be 5 mm less deep with a thermal resistance of 0.3 °C/W. My heat sink is on one side 250 mm long and on the other 70 mm long. That should estimated get me somewhere around 120-140 with 30°C dT. If I can keep thermal losses low enough from the die to the heatsink it should theoretically work out.
What should work in my favour is that my heat pipes are distributing the heat along the height of the heat sink and there also 3 dedicated heat pipes to the front where there is more heatsink surface. What works against me is that some heat pipes are rather long at 250 mm. However there is additionally also a pretty large direct aluminum link to the heat sink which should also transfer some of the heat.
That's my reasoning. How well I estimated it all i will only see once I have it completed and I can test it.
Manufacturing
Ok, I possibly did not go the most effective route and most definitely not a cheap one. But I have to say I did enjoy to design CNC parts and see them turning out just as I wanted them. I was already aware that my design with slanted fairly narrow fins (for CNC) would probably need more advanced tools. I was ordering it form JLCCNC and they used wire cutting to cut the heat sink fins and then milled the rest. That came at a rather steep price admittedly. But at least I got app. 5 kg heat sink in return for it.