Many K'NEX sets arrive in plastic storage tubs or robust cardboard boxes, which can be used for storage of the set when not in use.
However, for the serious K'NEX user, we would always recommend storing your K'NEX parts in compartmented storage trays, such as the one shown in the photo.
These are manufactured by companies such as Stanley, and can be bought in many countries worldwide from DIY stores and Hardware stores such as (in the UK) B&Q and The Range.
In our experience, keeping different K'NEX parts in different compartments makes designing and building K'NEX models not only quicker, but also more enjoyable.
An alternative approach is to use a robust plastic storage tub with tray inserts, such as the one shown in the photo.
This is a more elegant though more expensive approach than the compartmented tray shown above.
The items shown in the photo include:
9 litre Really Useful Box (available in different colours)
9 litre sorting tray (7 compartments)
They can be purchased from Really Useful Products Ltd.