Running Monterey, need to make a Mojave USB boot and/or install drive

I'm on a Mac Mini (2018), running Monterey. The Mini can run MacOS as far back as Mojave.

For $reasons, I need to create a Mojave boot drive. The Mini can boot from Mojave, which was the version of MacOS it ran when I bought it.

I have downloaded the Mojave installer (install macOS Mojave.app) to an external USB SSD drive. I would like to use this to install Mojave on another external USB SSD drive. But when I try to launch the installer, I get a message "This copy of the "Install macOS application is too old to be opened on this version of macOS.".

How can I launch the Mojave installer to make an external Mojave boot drive? My motivation is to save a database with 45 years of references, assembled with Papers3, which stopped running on Monterey sometime in the last two or three updates. I want a Mojave boot drive to regress so that Papers3 can run.

Thanks very much for any help you can give.