Its a bit more than a heat sink that’s needed. Apple’s physical design still won’t allow the heat to dissipate via the fan.
If I was Apple… I would have created a vapor chamber which can be made thin and then mounted with a fin assembly to the side with the fan blowing across pushing the heat out.
If you look you can see there is enough room to put one in. mmm… a possible upgrade kit??? You still would need to tune the thermal management logic within the SMC as Apples thermal curve still would be biased against the improved cooling.
References:
* [https://www.youtube.com/watch?v=M4POFT3Ctek|What is a Vapor Chamber as Fast As Possible]
* [https://www.youtube.com/watch?v=CCoLJeUbZTc&t=1530s|Intel PCIe PC Ghost Canyon Review: NUC Thermals, Noise, & Tear-Down]