I am totally confused with your question. Do you not have a cage that has a door? Simply shut the door and lock it, and the bird will stay inside.
Birds should not be eating porridge, unless you are speaking of hand feeding formula. Birds need to be eating high quality pellets like Harrison's or Roudybush and only 20% healthy treats. Don't feed table food, because the salt and chemicals are bad for them.
I don't know if your pet stores there carry high quality pellets, I sure hope so for your bird's sake.