Give Veigar's cage AoE damage on the inside.
This would work great:
Enemies inside the cage take 30/55/80/105/130(+15% AP) damage per second for a total of 90/165/240/315/390(+45% AP) and are slowed by 15% until they leave or the effect ends. Enemies are immune to the cage's damage and slow if they get stunned by the edges.