Looking into apt-key let me stumble upon something I always wanted to figure out… miscellaneous development log - #29 by Patrick Anyhow.
Once working please refine the apt-key command by adding --keyring /etc/apt/trusted.gpg.d/reponame.gpg. Much much cleaner than adding to /etc/apt/trusted.gpg! Also then instructions to disable it are as simple as sudo rm /etc/apt/trusted.gpg.d/reponame.gpg.