In Lutris there’s a “Command prefix” configuration option both per-game and in the global config with the dfeauot for all games which is where the firejail command line goes (basically sandbox with firejail you’re supposed to run “firejail firejail-options original-command original-options” and putting firejail and its options in “command prefix” does that).

Note that there are other sandboxing options that run in the same way as firejail but I found firejail to have more straightforward options.

Also note that this won’t sandboxes the actual setup of a game, only the running of the game.