UNetbootin allows you to create bootable Live USB drives for Ubuntu
and other Linux distributions without burning a CD.
It runs on Windows, Linux and Mac OS X. It's licensed under the GNU General Public License (GPL) Version 2 or above, the source code can be found on GitHub.
Here's a screenshot of the program taken from its website:
I was in the same situation some time ago and found only 2 solutions, one for Windows one for Linux (Etcher supports 3 major platforms though, I tried only on Linux):
Rufus and Etcher
Flash OS images to SD cards & USB drives, safely and easily.
Etcher is a powerful OS image flasher built with web technologies to
ensure flashing an SDCard or USB ...
If you don't mind installing a Linux system on your flashdrive you can do this rather easily using the grml-rescueboot package. I used Lubuntu for it's small size and flexiblitly, installed the grml rescueboot package and dropped the ISO's I wanted to boot in the /boot/grml folder and ran sudo update-grub grml-rescueboot adds scripts to generate the menu and ...
Too bad, I didn't see the question earlier. Steve Barnes's answer is completely wrong.
You can clone your drives, using any partition editor. Basiclly, the target drive has to be no less size, then the used space of the source disk. They sacrifice some free space from the target's partition, in order to fit the clone.
Some programs, that I can mention, that ...
Basically, you can't because two clones must be identical, that is what the word means, which results in the same apparent size.
The quickest and easiest answer is to procure a batch of 20+ identical USB drives, (you may well be able to get a price break at that quantity) - then use any of the existing cloning software including the one you have found.
You could use YUMI, it creates a good menu and places entrys for each Iso loaded!
You can get it here for windows:
YUMI multiboot pendrive
It really works great with Linux distros and various rescue discs, so it should work with Windows OS'es too.
Though this is not a software recommendation, but - IODD hosts a lot of ISO images, bootable or not. Some models support also multiple "virtual disks".
(Older USB2.0 models a.k.a. "Zalman Virtual CD emulator")