Easiest way to do it is indeed Win32 Disk Imager to image Kali to a usb disk. Use new Kali Linux Live USB to boot system launch gparted to create a new partition (use all unallocated space) with ext4 as file system with label persistence then use the following commands.
mkdir /mnt/usb
mount /dev/sdc2 /mnt/usb
echo “/ union” >> /mnt/usb/persistence.conf
umount /mnt/usb
Substitute /dev/sdc2 with the correct drive location of your usb. Most of the time its sdb3 but you still need to verify this though through gparted or dmesg.