Shouldn't be too difficult to do. I would change your schema to this:
if you use multi-key joins, then you can, if needed, display just the barrels included for the shipping rec, etc.
now for your UI, couple of options. You could have a clean UI that when you land on a Shipping rec, you then have a portal for Containers, limited to display just up to 3 lines. The user would create the Containers first. Then, they would select the container row - you need a script that sets the ID_Container key value and the ID Shipping key value to global fields stored in the Shipping rec. The global fields would be part of a join from Shipping to Flatrack and then the user would be able to add the flat rack records. Then, after adding the flat racks, selecting one would set an additional global in Shipping that stores the select ID_Flatrack, and activates your next join based on those 3 globals to your Barrel table, and the user can then enter Barrels for the selected Flat rack.
Now, if they are insistent on having distinct portals for each Container, then you just need to create distinct globals for each TO you need to create, create the TOs and then you have the appearance of each as separate, indepenedent portals on your layout. With the benefit of everything being easily reported on.
Sounds complex, but in practice its not.
thanks so much!! I was overthinking it a bit and I think this is the best approach!!!
glad to help! I've built a few UI's along these lines so happy to share.