customize live cd and add custom user name with default password

For some professional reason, we need to make our simplified live ubuntu version.
(It has to be live, can't be installed on flash drive)

In this link they explain how to disable the auto login, https://help.ubuntu.com/community/LiveCDCustomization#Removing_the_.28Casper.29_Autologin
but now I need to create a default username and password, so how can I achieve this?

Thanks in advance

I found a a solution using cubic.
In case anyone is looking to achieve this:
1-I installed cubic, followed the usual steps(create a project folder, select iso...etc).
2 - In the chroot env. I used vi to edit casper files and disable autologin
https://help.ubuntu.com/community/LiveCDCustomization#Removing_the_.28Casper.29_Autologin

3-after I exited vi, but still in the chroot environment, I added a new user as usual with useradd mycustomuser then add a password with passwd mycustomuser

Then I clicked next and followed the cubic wizard, burned the iso to a flash drive and it worked

Now every time you boot in live mode, it will ask for the password we created. Also if you decide to install the installation will have this pre-created account by default