There is sure method to trap wither, but it is costly and I really don't want to spill the beans Think a bit BTW trapping wither in bedrock is no longer good method, as wither is not spawning if the soulsand figure blocks touches anything else than floor.
Although I haven't tried it, I believe there is a bedrock box north of frontier spawn on smp7. edit: Also, that community you mention, is it yours? Because it would be griefing to summon a wither next to someone elses build
It's generally a bad idea to summon a wither within a few hundred blocks of any building, especially if it's not yours, because if things get destroyed it's on you. You're only around 20 blocks from someone else's building.