Step 1: Follow this to create Multi-Boot USB
Step 2: Use IMGBurn to create ISO from files. Just select the USB drive.
--Keep a copy, this is the untainted Easy2Boot LiveOS, which can be used to image USB's without OS's and its 500MB. We will create a large Multi-Boot ISO with all ISO's inside as well.
Step 3: Unpack ISO with 7Zip and then add ISO's to corresponding directories. I like to add some programs to ISO like Rufus, incase Easy2Boot fails, you can convert USB to just the LiveOS needed. Currently though the only issue I have had with Easy2Boot is Windows 10, and its very new ATM and I expect it to be fixed, but who knows.
Step 4: Recreate a NEW ISO with IMGBurn same as before, but selecting the folder previously extracted and modified.
--You now have your multi-boot ISO, only downside is to change it, it must be repackaged as stated above. I have tried ISOMaster but Rufus complained the new ISO wasn't bootable. Perhaps ISOMaster is better for modifing downloaded ISOs but IMGBurn is better if you create and modify the ISO. I just know what worked!
Step 5: To test and deploy use Rufus. The test part of this, is that if the ISO is not bootable Rufus will refuse to image it to the USB.
Check out Test LiveUSB in VirtualBox for troubleshooting and testing of new ISO!