I love these projects and I have really tried to squeeze my terrain collections into easily stored spaces as well. I don't (yet?) have infinite space for storing my collection of things, so I've really worked hard to tightly pack into the space I have rather than expand beyond that point. I'm not to the point where I'm ready to flat pack terrain, but I do design with the idea that things have to stack, fit a certain box, or nest inside something else. Starting a project usually begins with "what size space do I have open? in this storage container?"
Great work there. The ruined buildings look great on top of being storage-friendly. For foam buildings I would suggest a coating of spackle/PVA as it both covers some of the foam ball look (not necessary, but a nice touch) and creates a hard shell so the foam bits don't get snagged or damaged during storage and play. That extra durability is priceless for terrain that will go in and out of a box on a regular basis.