I'd be interested in exactly how do you add those pennies to your containers?
As I posted, I take the containers apart. Older ones are not glued and are easier. I noticed that newer ones from Walther's are glued and have to taken apart gently using an Exacto knife or a single-edge razor blade.
Here's an example of a 40' Athearn container using Walther's Goo to hold the 14¢ inside.
I use half as much on 20' containers.
On this one I added a piece of styrene down the center since the outer ribs are a little far apart.
Here's a Walther's 40' container that has no ribs on the floor.
Since on my layout, 53' domestic containers normally go on top, they get only 9¢. Keeps the loads from becoming too top-heavy.
I do two other things to all containers. I drill out the paint/plastic from the four small holes in the top, so they nest together more easily. And I use Woodland Scenics "Scenic Accents Glue" when reassembling. Keeps the bottoms from falling out when I rotate containers from one car to another. Since it is "repositionable" glue, it's easy to take them apart again if anything comes loose. That only happens if I used super glue to attach the pennies to begin with.
Things that I have observed on the trains coming out of the BNSF Alliance Intermodal Yard over the years. Most cars have 40' containers on the bottom. 20' containers go on the bottom. If stacked at all, they are only stacked on other 20' containers. They are never stacked on anything longer. Fully loaded 20' tanks normally have nothing stacked on top of them due to the weight of the liquid in them. They never stack a 40' container on top of a longer one, whether it be 45', 48', or 53'. 45 footers are pretty much obsolete any more. International containers are only 20' and 40'. Domestic containers can be any length, but the load bearing pillars are always 40' apart.
Hope that this helps.