As I am about to assemble my construction (Raspi 3B with 2 HATs on top + tuner + amplifier board next to the Pi) into a fairly tight (18 cm × 11 cm × 4.5 cm) metal enclosure, which will be then built into a car dashboard, I'd like to know how to lead the excess heat of the Pi's CPU to a massive aluminum heatsink, which is located some 9 cm away from the Pi's CPU on the left side of the enclosure. The amp and the regulators touch the heatsink and are therefore not problematic, but the Pi and the heat it produces might be an issue: As the Pi will decode and play MP3 files, CPU load may grow quite high for extended periods of time.
So: What should I employ to get the heat the Pi produces to the heatsink? The only solution I've found is copper tape (1 cm wide) attached both to the heatspreader of the CPU, and the heatsink. Would that be sufficient?