
Work to ride!
The box seems less than secure in the second photo, but you're clearly more experienced with bulky cargo than I. I've had problems toting long skinny items, like wiper blades and cutain rods. Still, I'll recommend a pair of
ROK Straps.
On my previous bike I had a big
Rocket Locker tail bag that replaced the passenger pillion, and it allowed me to do grocery runs with relative ease. Loved it because it didn't change the footprint of the bike.
When dismounting, do you
always remember there is a big box behind you? Pretty sure I've fallen on my butt at least once after my airborne
right* foot discovered forgotten cargo.

*EDIT, the right foot swings over the seat, not the left. d'oh!