Do you have a tutorial to repackage non-DRMed games for flatpak, for personal usage? Could be very useful!

No tutorial, just organically grown (thus ugly) scripts for games, that I own.

The gist is to prepare the directory structure, icon, .desktop file, flatpak metadata, appstream info, add dependencies not provided by flatpak runtime (for games, libGLU for example), put everything into ostree repo (flatpak build-export ...), and then either install from that, or export the content into .flatpak file (flatpak build-bundle ...).

The last step can take a while, .flatpak files are lzma compressed and the compression during export uses only one cpu core. For 20GB game, that isn't exactly quick, to be diplomatic.