I'm contemplating on converting to using the Raaco system for most of my storage, but I'm kind of missing one component, and that would be a lid/base grid combined so that I could stack multiple levels of boxes on top of each other. Would that be hard to add to the pack?

I'm thinking like combining the 5x5x4 lid with a Grid 5x4, potentially loosing a box at the ends for a handle, or something like that. Such an element would allow me to have multiple levels of smaller boxes which would be very useful for the more longtime storage, and still allow for using all of the smaller boxes for the individual items.

Perfect! I plan to use it for organizing my drawers similarly to the gridfinity system.