Ah, found yourself an empty spool? A plastic one from Esun (Or any similar one) perhaps? And ever wondered if you could have a bit more extra storage somewhere? LOOK NO FURTHER!!! I have EXACTLY what you NEED !!! 

Two storage compartments hidden(ish) inside a filament spool with magnets for a satisfying closing mechanism. Besides that, the compartments have a small part on the top of it which perfectly fits inside the long grooves in the spool so the compartments can slide in and out without randomly detaching. 

While closed, it almost resembles a spool which still has filament on it. But then you open it and find out there's a lot more to it than that! Store your pens, building bricks, or even trash inside of it. The possibilities are endless! 

When slid out, the compartments (Or I guess you could also kinda call them drawers) can also rotate side to side a bit so that all the contents are easily accessible. And when you really need full access, you can also always take them out with little effort.

During the print, you will have to insert four D4x2mm magnets in the small little boxed for them. Make sure to place them in the right orientation so that you won't end up having two compartments that push each other away instead of attracting each other!