There is a new tool available for Sparkers: MultiBootUSB.

What is MultiBootUSB?

MultiBootUSB is a software/utility to create multi boot live Linux on a removable USB disk. It is similar to UNetbootin but many distros can be installed, provided you have enough space on the disk. MultiBootUSB also provides an option to uninstall distro(s) at any time, if you wish.

MultiBootUSB allows you to do the following:
– Install multiple live Linux and other Operating Systems to a USB disk and make it bootable without erasing existing data.
– Ability to uninstall installed OS later.
– Write ISO image directly to a USB disk (you can think of GUI for Linux dd command).
– Boot ISO images directly without rebooting your system using QEMU option.
– Boot bootable USBs without rebooting your system using QEMU option.
– Boot USB on UEFI/EFI system through GRUB2 bootloader (limited support).

Installation:
sudo apt update
sudo apt install python3-multibootusb

The MultiBootUSB GitHub project page: github.com/mbusb/multibootusb
The project author is Sundar; and co-author is Ian Bruce.