For anyone else reading through here for advice on boxes. Two pieces of advice.
1. Check out Harbor Freight, their new boxes are over built even compared to just a couple of years ago. They now have to because of Import Trade Agreement. This is why they are deeper and made with heavier gauge...
I just got a HF 44" top and bottom during their parking lot sale. Considered the 56" and also did a lot of searching for used tool truck boxes as well as researching on which boxes seemed to last and we're reasonably priced. Decided on the 44". If I need more room I'll buy another 44" base...