These are some toy cargo pallets that I created for my children to play with. They fit on 2x2 and 2x3 Duplo blocks, or can be used on their own.
Each block has a hole centered to attach an eye hook so the blocks can be picked up with a toy crane, for loading and unloading play.
I have also included a blank pallet so you can create your own

This object was made in Tinkercad. Edit it online https://www.tinkercad.com/things/0SMHHvRQX08