Comment 1 for bug 1623261

Hello.
I find how to resolve this issue.

Need to install syslinux-utils and run in terminal $ sudo isohybrid /path/to/custom_iso.iso
 or add to utilities.py new string:

hybrid = 'isohybrid "%s" .' % (
        model.custom_iso_image_volume_id, model.custom_iso_image_filepath)
    execute_asynchronous(thread, hybrid, model.custom_live_iso_directory)

But this solution don't work for UEFI mode, i can't create second partition (EFI) in .ISO file:

ubuntu-16.04.1-desktop-amd64.iso1 * 0 2955679 2955680 1,4G 0 Empty
ubuntu-16.04.1-desktop-amd64.iso2 2927216 2931951 4736 2,3M ef EFI (FAT

Maybe this information can help you for update CUBIC Repackage Functions