Go to any settlement with settlers in it and build the following four items:
- Arena contestant platform (red)
- Arena contestant platform (blue)
- Quitting time siren
- Power source for the quitting time siren. A small generator should do
Assign a person to each platform. Followers free roaming the settlement are recommended as they cannot die.
Come out of build mode, then activate the quitting time siren.
This should result in the two people fighting and your other settlers gathering round to watch.