I created this shelf so I had somewhere accessible to store longer tools like 18inch breaker bars, pry bars, and extensions. Once assembled the shelf can simply be lowered into place.

The shelf leaves plenty of space at the bottom of the box for miscellaneous items and still has plenty of clearance up top for the half width tray insert that comes with the STACK system.

EDIT: After a winter in the garage, the shelf (Printed in PLA+) became a little loose. I've added an extra wide join piece with more support to reduce this
 

PRINTING INSTRUCTIONS:

This shelf can either be printed in 3 pieces if you have a very large printer (>425mm) or 5 pieces if you have a printer with build volume wider than 210mm.

Shelf bases-
Both of the shelf side bases are designed to be printed with the tapered side down and the slot facing upwards

Shelf surface-
The shelf surface should be printed with the large flat side down so the groove on the back and the lip on the front are pointing up

Shelf join-
The shelf join piece is designed to force bridging in as way that makes supports unnecessary, print it on either side so that one slot is facing down and the other slot faces up
 

ASSEMBLY:

Simply slide the shelf surface pieces into the the shelf base slots, and slide the other side of the shelf bases into each side of the shelf join piece. 

I recommend using hot glue or an adhesive inside the shelf join slots so that the shelf doesn't pull apart slightly and sag when heavier tools are placed on the shelf.