You can’t go wrong with something like this!

In all seriousness though, I haven’t been able to get a root file system to compile with the guide so I cant say for myself about the issue, though if it is indeed overheating when under intense load, a plain heatsink will only delay the issue unless it is massive. Small aluminum heatsinks can provide a heat buffer for spikes in thermals, but anything continuous wants a fan.
The GPIO header has 3.3v power, so you could power a small 3.3v fan, (just be careful that the fan you choose does not draw enough current to damage the board. Unfortunately I don’t know how many mA the 3.3v line is capable of.) or you could use a molex to standard computer case fan adapter and plug into the onboard molex plug for the power for the fan.

Any amount of airflow should be enough to keep it cool, without a heatsink. Unless the heatsink has particularly good thermal transfer and a good amount of surface area on the fins, you will likely see better thermal performance without it if you use a fan.