The best thing you can do for these cheap S905X or S912 boxes and dongles is run them for a while and then open them up and twist or remove the heatsink off the SoC. Then clean up the SoC and heatsink with something that does not leave a greasy residue, like electrical Contact Cleaner.
Most heatsinks look like they are just fixed on with some sort of Silicon which makes for pretty bad heat transfer.
Then simply get some 30% Silver (or better) thermal paste, you only need tiny amounts and use this Sandwich Wrap method for applying the paste:
How to Apply Thermal Paste - iFixit
Just did this yesterday to a MECOOL S905X that would get pretty hot, especially when using Android and it now runs 10 - 20 Degrees C cooler as a result AND I can increase the GPU Mhz for a snappier, smoother LE Kodi user interface !
Some LE Temp monitoring tips:
SSH Login and....
echo "cat /sys/class/thermal/thermal_zone0/temp" > temp
chmod +x temp
watch ./temp
and I use this for the GPU:
echo "echo 2 > /sys/class/mpgpu/scale_mode" >> /storage/.config/autostart.sh
W.