I have started on the new WW1 project.
I wanted to get as much as possible in each box, and thought doubling the space would be a way. Basically cutting box in half(almost) the extending other half using the lid as a base. I don't intend to connect each half with track, but might be an idea for another project.
Each half can be connected to other modules. It might even be possible to connect something to both at same time.
Hopefully this will allow me to get even more layout into my small car. 3 or 4 APA boxes is mazimum
I plan to fill up space between tracks with various WW1 vehicle I have. Being kit based they tend to get broken if moved around so need a more permanent home. This one will be lorries, another will be tanks and a third will have an airplane. There will be a slightly twisted them to each,one aim being to make people think about war and railways.
I have had to use a new source of styrofoam. Not so keen on it as had to remove metal coating. Also don't think it is as strong. Should be OK for this project.
Height is not so much an issue as there are no tall buildings. Some vehicles might stick out, but by tinkering with positions, there should be no problem when folded up. Main thing is to keep models safe.