We built a wither arena now. You can go to spawn and then type /warp witherarena.
You will arrive in a safe room where you can gather. There you can go down a water hole and then walk over (follow the signs) to the wither spawn box. In that box, you can build a wither. In the rest of the arena building is disabled. This is the only place in on the whole server where you can actually build a wither. You can go in there with as many people as you like.
Please do not barge in on people killing the wither. Check if the people fighting it allow you to join before you steal their kill. If someone spawns a wither however and fails to kill it, anyone else coming to the arena while nobody is in there can have it. you cannot claim it for the next 3 days just because you do not have time to kill it.
If that rule fails, there is one possible addition: We can create a system so that people create a team that will fight the wither and that will block everyone outside the team from entering the arena for a certain time (Let’s say 1 hour). This is quite a lot of work, so I will do that only if it is necessary.