I put my topbox on and off all the time. I personally hate it as it ruins the classic lines of the Vespa, but my wife feels much safer when we ride 2-up and it's just so damn practical. I've fitted two wing-nut bolts that can be undone by hand and I keep a screwdriver handy to quickly remove the other two bolts. Whole process takes less than 2 minutes.
I seldom remove the black clamp and this just holds the rack down so I can strap items if I need to.
Only downside is the bolts now protrude into the box, so I've lined the base with acoustic foam. This prevents things from sliding around and also stops fruit from getting bruised.