The grub.cfg could be used to make a multiboot live USB.

grub.cfg

function load_efi_video {
    set gfxmode=auto
    insmod efi_gop
    insmod efi_uga
    insmod gfxterm
    terminal_output gfxterm
}

function load_video {
    set gfxmode=auto
    insmod vbe
    insmod vga
    insmod video_bochs
    insmod video_cirrus
    insmod gfxterm
    terminal_output gfxterm
}

if loadfont ($root)/boot/grub/fonts/unicode.pf2 ; then
    if load_efi_video ; then
        set bios="UEFI BIOS"
    else
        load_video
        set bios="Legacy BIOS"
    fi
fi

set locale_dir=($root)/boot/grub/locale
#set lang=zh_TW
insmod gettext

set menu_color_normal=white/black
set menu_color_highlight=black/light-gray
if background_color 44,0,30; then
    clear
fi

set gfxpayload=keep

#set options="file=/cdrom/preseed/ubuntu.seed boot=casper quiet splash locale=zh_TW.UTF-8"
set options="file=/cdrom/preseed/ubuntu.seed boot=casper quiet splash"


menuentry "GParted ($bios)" {
        set isofile=/iso/gparted-live-0.28.1-1-amd64.iso
        loopback loop $isofile
        set root=(loop)
        search --set -f /live/vmlinuz
        linux   (loop)/live/vmlinuz boot=live union=overlay username=user config components quiet noswap ip= net.ifnames=0  nosplash iso-scan/filename=$isofile --
        initrd  (loop)/live/initrd.img
}

menuentry "Ubuntu 18.10 ($bios) dailybuild" {
        set isofile=/iso/cosmic-desktop-amd64.iso
        loopback loop $isofile
        set root=(loop)
        linux   (loop)/casper/vmlinuz iso-scan/filename=$isofile $options --
        initrd  (loop)/casper/initrd.lz
}

menuentry "Ubuntu 18.04 ($bios) Bionic Beaver" {
        set isofile=/iso/ubuntu-18.04-desktop-amd64.iso
        loopback loop $isofile
        set root=(loop)
        linux   (loop)/casper/vmlinuz iso-scan/filename=$isofile $options --
        initrd  (loop)/casper/initrd.lz
}

menuentry "Ubuntu 16.04.5 ($bios) Xenial Xerus" {
        set isofile=/iso/ubuntu-16.04.5-desktop-amd64.iso
        loopback loop $isofile
        set root=(loop)
        linux   (loop)/casper/vmlinuz iso-scan/filename=$isofile $options --
        initrd  (loop)/casper/initrd.lz
}

menuentry "Ubuntu 16.04.2 ($bios) Xenial Xerus" {
        set isofile=/iso/ubuntu-16.04.2-desktop-amd64.iso
        loopback loop $isofile
        set root=(loop)
        linux   (loop)/casper/vmlinuz.efi iso-scan/filename=$isofile $options --
        initrd  (loop)/casper/initrd.lz
}

menuentry "Ubuntu 16.04.1 ($bios) Xenial Xerus" {
        set isofile=/iso/ubuntu-16.04.1-desktop-amd64.iso
        loopback loop $isofile
        set root=(loop)
        linux   (loop)/casper/vmlinuz.efi iso-scan/filename=$isofile $options --
        initrd  (loop)/casper/initrd.lz
}

menuentry "Ubuntu 16.04.1 ($bios) Xenial Xerus - Unattended Installation" {
        set isofile=/iso/ubuntu-16.04.1-desktop-amd64-unattended.iso
        loopback loop $isofile
        set root=(loop)
        linux   (loop)/casper/vmlinuz.efi iso-scan/filename=$isofile file=/cdrom/preseed/unattended-ubuntu.seed boot=casper automatic-ubiquity quiet splash languagechooser/language-name=English debian-installer/locale=en_US keyboard-configuration/layoutcode=us --
        initrd  (loop)/casper/initrd.lz
}

menuentry "Ubuntu 14.04.5 ($bios) Trusty Tahr" {
        set isofile=/iso/ubuntu-14.04.5-desktop-amd64.iso
        loopback loop $isofile
        set root=(loop)
        linux   (loop)/casper/vmlinuz.efi iso-scan/filename=$isofile $options --
        initrd  (loop)/casper/initrd.lz
}

menuentry "Ubuntu 14.04.1 ($bios) Trusty Tahr" {
        set isofile=/iso/ubuntu-14.04.1-desktop-amd64.iso
        loopback loop $isofile
        set root=(loop)
        linux   (loop)/casper/vmlinuz.efi iso-scan/filename=$isofile $options --
        initrd  (loop)/casper/initrd.lz
}