There are several multiboot applications available and I have used most of them. After making one from all of them I settled on Easy2Boot. This software seems the easiest to use and has a lot of bells and whistles built-in. No matter which application you choose to use, the most time you will spend is downloading the relevant ISOs. My Second favorite app is SARDU. I worked with the developer to help bring the latest version out (he did all the coding and I made a lot of suggestions and did testing). SARDU has a better way to do the downloading than Easy2Boot does, but when I built the SARDU multiboot USB stick, the interface was better with Easy2Boot.
What I sometimes end up doing is downloading the iso files with one app and building the USB stick with the other (I have used both to download and both to build the stick. I find it easier to test the stick’s functionality using Easy2Boot). Note that neither of these methods can be used to boot a machine that has serious encryption. As the rest of this site is, so are these aimed at the purely personal computer user).
Easy2Boot does have a limit on stick size (128gb).