The original inserts didn't quite fit my needs, there were some play. So I remixed it to fit perfectly in a small SBox.
Changes: The bottom grid corner towers are raised by 2mm to accomodate the full height of 3u gridfinity boxes and the top stackable insert is thickened by 3mm and half a mm is cut from all four sides. This creates a snug fit height wise in the small SBox, and a not too thight fit for the top grid in xy-dimension. I've also made the insert solid so it doubles as a lid for the lower level.
Print a small SBox, these two files and your selection of 3u and 6u gridfinity boxes.
The author remixed this model.