User can launch any of the program emulators (microcode-level firmware interpreter).
Application icon shows-up in desktop launcher after install.
Right-click on icon to set default model for direct access, or launch a sample model from list.

Application may be started under terminal (to monitor eventual error messages):
flatpak run io.github.mike632t.x11-calc (add --setup to set defaults)
Defaults may be edited manually from host:
nano ~/.var/app/io.github.mike632t.x11-calc/config/x11-calc/x11-calc.conf

Application is strictly sandboxed, so it can only process files (*.dat, *.rom) within host's:
~/.var/app/io.github.mike632t.x11-calc/data/x11-calc/
which x11-calc application can access there, or within sandboxed /var/data/x11-calc/.
Similarly some read-only sample saved programs are in sandboxed /app/share/x11-calc/prg/.

Install flatpak-builder. Copy / clone this repo in a directory and then run:
flatpak-builder -v --force-clean build-dir io.github.mike632t.x11-calc.yaml
Test install:
flatpak-builder --user --install --force-clean build-dir io.github.mike632t.x11-calc.yaml

Once ready, extract side-loadable x11-calc.flatpak package for later distribution:
flatpak build-bundle ~/.local/share/flatpak/repo x11-calc.flatpak io.github.mike632t.x11-calc