UNTESTED
1. go to the arena and wait.
2. right before the chest appears (like 5 sec) go to the place, where it will appear and use a item that shrinks you (e.g. a noggenfogger). it helps, when you go there stealthed, so nobody can attack you, while you go there.
3. when everything works alright, you are now standing in the chest and nobody can attack you (except with /target YOU), because when they try to target you they can only click on the chest.