It reall depends on what you consider custom plans.
To me Pre-made plans would consist of a rectangular shapes with nothing out of the ordinary. If I already have cage plas for 4' X 2' X 2' cage, then to make a 4' X 6' X 2' I can modify the existing plans to suit that size of cage. I would consider a custom cage to be something with a base and a canopy, maybe a bow front, hinged doors as opposed to typical sliging glass doors.
Quality Customized Terrariums