Keeping the toolbox and just adding a wing to rear of the cab might give you almost the full benefit of a full aero-shell with much less work:
__________________
2016: 128.75L for 1875.00km => 6.87L/100km (34.3MPG US)
2017: 209.14L for 4244.00km => 4.93L/100km (47.7MPG US)