There is a flat walled and a ridged walled trailer box. You only need to print one of the boxes.

It will work with the trailer depot. 

For the coupler: https://www.printables.com/model/1049997-auto-world-to-tyco-us-1-truck-hitch

For the wheels: https://www.printables.com/model/1170849-tyco-us-1-plastic-wheel-and-tire

I used a 27.5mm long by 1.2mm diameter axle: https://www.slotcarcentral.com/mm5/merchant.mvc?Screen=PROD&Store_Code=SCC&Product_Code=8AAGGTUPUSGAROE&Category_Code=

The pivot on the truck is long. The spacer on the trailer was originally taller. I saw no reason to shorten it. 

The holes will need some finishing. I lubricated the pivot point. The coupler hole is tight. With my prints, the box fits snugly on the chassis. Results may vary, you can glue it o, if you want. I have printed it in FDM and Resin, and it works with each. I prefer to print the truck and chassis in resin and the box with FDM. The doors do not open.