You're imprisoning embodiments of the primal forces of reality. Personally, I would feel something nearly equally primal would be needed. To oppose all four of the cardinal elements, let us turn to the intercardinal elements, the combination of the ideas into something new.
From Fire and Earth, mortal man smelts the power of metal, at once the bite of the sword and the bulwark of the shield.
From Earth and Water, Nature works her leafy miracles, from the lowly shrub to the great redwood, the power to grow and thrive. Mortal man names this power Wood.
From Water and Wind, the spirits of the past and present bring forth the power of the Mist, the concealing veil between places, the power that clouds, confuses, and chokes.
From Wind and Fire, the Gods forge their wrath, the electric flame of the skies that man names Lightning.
We begin, then, by forging mortal man's contribution to the prison. In every shape save one, there are weaknesses, corners and edges to pry and defeat. That shape is the sphere. The sphere is significant in size, a great, open cage, free of weldings and seams, that hovers majestically above the ground.
Nature, then makes her contribution, lacing the gaps of the cage with the ageless resilience of the most ancient forests, great, leafy vines interwoven with the struts of the metallic sphere, taking support and strength from the metal, nourshed by, and in turn, nourishing the third component of the sphere.
The dead fill the gaps that the living cannot, with blinding, haunting mists that echo of sorrow and sadness, though at the same time, it nourshes the great vines and carries the final component of the sphere.
Finally, the gods have given their blessing to the scheme, enthreadind the sphere with thundrous, crackling lightning. It courses widly through the mists and framework, eternally reforging, strengthening the struts.
Far more than a mere prison, there is an insidious quality to this great enchantment: It draws and feeds upon not the wills of the casters, but rather, upon its prisoners themselves. At once both ambrosia and anathema, the elementals cannot help but attempt to merge with, to 'purify' the combined elements of the sphere, yet always they find that they are the ones consumed, integrated into the combinatorial magics. Far, far more power is consumed than is needed, and over time, this power is crystallized by the forces within the prison, let fall to the wind and weather: The sands of Aros. It was the hope of the magi that this sand would return to nature, distributing its life and power again to the barren world, free of the malevolant intelligence it once possessed...