There is a simple yet effective way to get large amounts of chests, boxes, crates or barrels close together, normally you can only get around two objects right next to each other but if you use this method you can get many objects close together.
First, you make an elevation where you can place these objects. I have a theory that the higher the elevation is, the more objects you can place.
Now place as many of the desired object as possible, and remember to check all 360 degrees of the circle around the object.
You can add more by placing them just anywhere on the slope and dragging them to the original group. sometimes, you need to drag the original group to the new group for it to work. Also, it seems to work best if you drag from the bottom.
Lower the elevation of the entire hill to whatever you wanted it to be.
Here's an example:
Grouping objects together like this can be very helpful to your map design, especially for cinematics. Have fun!