As in thermal image, hotest chip is not CPU, but LAN chip! and also Wifi dongle!
Anyway, CPU is a thing worth for cooling.
With heatsink, Raspberry Pi will decrease CPU temperature about 5 dgC (from 45 dgC to 40 dgC)
This is a passive way without any power consumption. How to make it?
Step1. Prepare a aluminium heatsink
Step2. Prepare heat silicone (other name: thermal grease)
Step 3. Coating a little heat silicone to CPU and heat sink
Step 4. Stick heat sink to CPU
Step 5. Using cable tie to fixed them all.