Last small job - place the box in the van and screw the base of the box to van's floor. Use many screws so the box will not move when driving.
Step 5: Box base cover
Screw support for the inner 2*2 at 45º on all sides (in my image you see only one side), use glue for extra strength.
After cutting plywood to size screw it to the box base. The base will be used to attach the box to the van's floor so don't be "shy" with the screws when attaching the base plywood.