Linux Kali - persistent storage

This is my first experience with Linux and I have a Kali Linux Live CD on my external HD. I followed this tutorial to set up an encrypted and persistent partition.

my partitions at the moment

/dev/sdb1 
/dev/sdb2  // file system
/dev/sdb3  // persistent storage ( Crypt-luks)

Now I wish to change my MAC address on each boot, so I added this to ect/netowrk/interfaces

ifconfig wlan0 down
macchanger -r wlan0
ifconfig wlan0 up

I can see the interfaces file has been created and updated in /lib/live/mount/persistence/sdb3/etc/network

The problem is that whenever I reboot my pc the commands I saved earlier are erased, and I basically find just an initial duplicate of interfaces