A box works but the problem with a box is that when the seedlings grow, you need to adjust the height of the fluorescent light to provide the 4 to 6 in clearance. You can switch to a taller box but a crate is a better alternative. In a real green house, the fluorescent lights are hung using adjustable chains from the ceiling. A crate has slats that can be used to hold cardboards to create different heights. As the plant grows, the cardboards can be moved to a lower level.