Custom multi-boot USB
Insert USB memory stick
Open Terminal
Find USB using lsblk (eg /sda/sdX)
sudo umount /dev/sda
sudo mkfs.exfat -F /dev/sdX
sudo mkdir -p /mnt/usb
sudo mount /dev/sdX /mnt/usb
sudo grub2-install --target=x86_64-efi --efi-directory=/mnt/usb --boot-directory=/mnt/usb/boot --removable --force /dev/sdX
sudo mkdir /mnt/usb/iso
Change to the ISO files directory
sudo cp *.iso /mnt/usb/iso