Cage

You create a 5 foot cubed cage of mystical energy in the space occupied by a medium or smaller creature. The cage can contain one creature of Medium size, 2 creatures of Small size, or 4 creatures of Tiny size, provided that they are all within 5 feet of the target space. The cage appears within range and lasts for the duration or until dispelled as an action. If cast in mid-air, ie. around a flying creature, once fully formed the cage and its contents fall to the ground, both suffering 1d6 Bludgeoning damage. 

When the cage appears, each creature within its area must make a Dexterity saving throw. On a failed save, a creature is restrained within the cage. Its movement speed is reduced to 0 and it cannot make melee attacks to anything outside the cage. On a successful save, a creature evades being enclosed in the cage and the magic dissipates. 

Once formed, the cage behaves like mundane iron cage. A creature can use its action to attempt to bend or break the mystical bars with a Strength DC 18 check. On a success, the bars break and the cage is dispelled. 

At Higher Levels. When you cast this spell using a spell slot higher than 3rd, the size of the cage increases by 5 feet per level of spell. 4th level = 10 foot cube, 5th level = 15 foot cube, etc. A 4th level cage would hold 1 Large creature, 2, Medium creatures, 4 Small creatures, or 8 Tiny creatures.