Jeremy Cook
Heat sink was removed from old Lenovo PC that I bought off-lease and ran my CNC router for some time, then cut to shape so it would fit
Used a milling machine here, but a band saw would have probably worked. Attached to raspberry Pi with thermal tape, and insulated overhang area with electrical tape to avoid shorts if it contacts anything else.
After application there was a significant drop in both ambient and working temperature. Checked using Putty with "vcgencmd measure_temp" command. Overall a simple hack, but effective.
Also used the case for this PC for my NAS build as seen here, so don't toss that old PC before scrounging for parts!
derekcaelin
JessupAdam
Definitely overkill on a Pi 3, but I have almost exactly this on both a Tinker Board and a Pi 4. I always pull heat sinks from electronics before recycling. Many old 2000's era motherboards had a 40mm square heat sink on the North Bridge that will sit flat on a Pi 4. No need for a fan as long as you aren't overclocking.