Why not have both options? This has both the tutorial for the truck, with the closed back, and the box, which you can add separately.
This tutorial includes 27 pages of instruction for the truck and 8 pages of instruction for the box, including color photos and diagrams of each step.
Truck finished size: 3.4"L x 1.1"W x 1.5"H
Removable Box finished size: 2"L x 1"W x 1.1"H