This is the first Mechanical thing I've designed, What a challenge!  I got my Prusa Mk3 for Christmas and started watching Fusion 360 tutorials on YouTube.  What an incredible Hobby!  

Assembly:( I've added Photos of the steps).  First start with the bottom Piece. Place the double gear on the inside pin with the larger gear at the bottom, then place the thumb gear on the outer pin. Put the tray on top, making contact with the bottom gear. To put on the lid; first line up the pin holes, then one at a time lock in the tabs on the gear side. Lastly, pull the lid over the locking tabs on the smooth side. You may have to work it back and forth a few times to break it in.  (If the pins are too tight the holes in the lid may need drilled out a bit.  They are designed to be snug)

I printed this in PLA at a .2 layer height along with 4 perimeters to insure the pins were solid.  The G-Code files are for all pieces printed at once (one with the blank lid and one with the design on the lid. If you want the color change you need to print separately because the height of the change is different on the tray and the top)

I will include the f3d and step files for the tray so that you can customize it for whatever you want to put in it and change the markings. I made 2 lids, one with a gear design and another that is blank so you can put whatever logo or design you like on top.