This is an inside nestbox. It hangs from the upper most corner of your cage. The boxes I use are 5X5X5 inches.

Here is an inside nestbox in one of my cages.

This is an outside nestbox. This nest hangs on the outside of the cage but for this you need to cut a small hole in the cage wire so the birds can get through. Note the hinged top so you can view your birds and babies. This box is also 5X5X5 inches.

This is the back view of the outside nestbox.