You should make a unit (I'd suggest a scout tower), then change the model file to the cage, give it and inventory under 'Abilities - Normal' and make sure it has no attack or anything like that (unless you want it to).
To make it respawn I would use this trigger:
Untitled Trigger 001
Events
Unit - A unit Dies
Conditions
(Unit-type of (Triggering unit)) Equal to Storage Cage
Actions
Wait 5.00 seconds
Unit - Create 1 Storage Cage for (Owner of (Triggering unit)) at (Position of (Triggering unit)) facing Default building facing (270.0) degrees
Although, the items stored in the cage before it died will be lost, maybe you can revive the cage instead of creating a new one.