Game servers by the hour. Pay only when you play.
Spin up a Sven Co-op dedicated server by the hour. Co-op campaigns for up to 32 players, no port-forwarding, friends join by IP, ready in ~5 minutes.
A listen server dies the moment the host quits. This is a real dedicated box, so bring a full squad and keep the run going.
The server sits on a public IP from the start. Friends just connect to the IP or find it in the in-game browser.
Around a hundred official co-op maps ship with the server and rotate on their own. Everyone already has them, so nobody waits on a download.