As an alternative or in addition to fabric pouches in your cage, nesting boxes can be made from a variety of materials, such as wood or plastic. Be sure to consider your material. Wood will soak up urine and require more cleanings.
They can be placed inside the cage, or attached to the outside with a hole cut in the side of your cage to allow your pet access to the box. If you have to cut a hole in your cage, check the edges and make sure to sand any burrs on wire or other cage materials.
Make sure your nestbox is large enough to allow airflow and consider fleece or sherpa blankets to add warmth, especially in winter. In some cases, webcams in the infrared spectrum have been attached to nesting boxes.
Nestboxes do not have to be complicated. Some examples: wooden nestboxes are often pre-made and sold in the bird aisle of pet stores; a Cool-whip bowl with a hole cut and secured to the side of the cage; a plastic box for index cards, also cut and secured.

Upgraded nest box. Copyright by: Catman, 2008

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Heating