A word of caution, the kernel module (iptable_nat) is not loaded permanently. After a reboot, it must be loaded manually again. (cc @mrmarkuz )
What would be the best/most efficient way to load the module at boot time using /etc/modules.d or /etc/modules or /etc/modules.conf etc.
using rc.local is not my desired route for I consider that as a last resort when all other methods fail.
edit:
For now I simply created /etc/modules-load.d/iptable_nat.conf with 1 line only
iptable_nat
Now the module loads automatically at boot time. However, I d agree with @davidep on: