The ISO should work for BIOS or UEFI and can be dd'ed to a USB device.

```
sudo apt-get install \
    isolinux \
    grub-pc-bin \
    grub-efi-amd64-bin
```

Create some dirs.

```
mkdir -p $HOME/live_boot/image/{boot/isolinux,EFI/boot}
```

Copy isolinux files.

```
sudo cp \
    /usr/lib/ISOLINUX/isolinux.bin \
    $HOME/live_boot/image/boot/isolinux/ && \
sudo cp \
    /usr/lib/syslinux/modules/bios/* \
    $HOME/live_boot/image/boot/isolinux/
```

Generate a `grub.cfg` menu configuration file.

```
cat <<'EOF' >/tmp/grub.cfg

search --no-floppy --set=root --file /DEB_CUSTOM_ISO.file

insmod all_video

set default="0"
set timeout=30

menuentry "Debian Live" {
    linux /live/vmlinuz boot=live quiet nomodeset
    initrd /live/initrd
}
EOF
```

Create a uniquely named file that we will use to infer our Grub root when we boot.

```
touch $HOME/live_boot/image/DEB_CUSTOM_ISO.file
```

Generate a UEFI boot image. You probably do not need all these modules. I ended up with this list after troubleshooting, and never trimmed the module list after I got things working. `grub-mkstandalone` is neat. It creates a self-contained file with everying Grub needs to boot, which is kind of nice since you needn't worry about paths, copying modules to the wrong spot, etc. This was most reliable for me across different hardware, but there make be a better way of doing this.

```
grub-mkstandalone \
    --directory="/usr/lib/grub/x86_64-efi" \
    --format=x86_64-efi \
    --output=$HOME/live_boot/image/EFI/boot/bootx64.efi \
    --modules="linux normal iso9660 efi_uga efi_gop fat chain disk exfat usb multiboot msdospart part_msdos part_gpt search part_gpt configfile ext2 boot" \
    "boot/grub/grub.cfg=/tmp/grub.cfg"
```

Create a FAT16 EFI disk image.

```
(cd $HOME/live_boot/image/EFI && \
    dd if=/dev/zero of=efiboot.img bs=1M count=100 && \
    mkfs.vfat efiboot.img && \
    mmd -i efiboot.img efi efi/boot efi/boot/grub && \
    mcopy -i efiboot.img boot/bootx64.efi ::efi/boot/
)
```

Generate an `isolinux.cfg` menu configuration file.

```
cat <<'EOF' >$HOME/live_boot/image/boot/isolinux/isolinux.cfg
UI menu.c32

MENU TITLE ISOLINUX Boot Menu
DEFAULT linux
TIMEOUT 600
LABEL linux
  MENU LABEL Some Linux Entry
  MENU DEFAULT
  KERNEL /live/vmlinuz
  APPEND initrd=/live/initrd boot=live quiet
  TEXT HELP
    Boot Linux
  ENDTEXT
EOF
```

Generate the ISO file. Note the `-isohybrid-mbr` flag, which allows writing the ISO to USB with `dd`.

```
xorriso \
    -as mkisofs \
    -iso-level 3 \
    -full-iso9660-filenames \
    -volid "DEBIAN_CUSTOM" \
    -isohybrid-mbr /usr/lib/ISOLINUX/isohdpfx.bin \
    -eltorito-boot \
        boot/isolinux/isolinux.bin \
        -no-emul-boot -boot-load-size 4 -boot-info-table \
        --eltorito-catalog boot/isolinux/isolinux.cat \
    -eltorito-alt-boot \
        -e EFI/efiboot.img \
        -no-emul-boot -isohybrid-gpt-basdat \
    -output "${HOME}/live_boot/debian-custom.iso" \
    "${HOME}/live_boot/image"
```