Prevent Enemies from leaving the Ultimatum circle.

Pretty much as the title says, its fucking annoying to say the least when i need to kill enemies to clear a wave, only for half the spawns to either start outside the circle so i cant reach them or (in the case of those spider bots) run away from you constantly and flee beyond the borders.

Could we get like a hard wall or perhaps a bigger area to stop enemies from just spawning out of zone/fleeing to it? I understand not everyone is going to have this problem, but for meele/ close range builds its annoying as hell.