I haven't got access to it right now, but it's very basic: Its a multi/sub-object material with ID#1 being Concrete (from "Corona Basic Set") for the box sides, and ID#2 being a Concrete/Fiber blend-material for the box front and back. As blend mask I used a Cellular map.
Regarding the "shadow-caster" I did try another way, that might let some subtle light inside (depending on your scene and the way your fiber material is build): Instead of one big vertical plane you can use a series of smaller horizontal planes placed above each other with a "building block size" between them. This will simulate the top and bottom of the building blocks in real life, and will block sun-rays from shining thru the wall, but allow horizontal light to pass.
I think the problem is, that unless you're building these walls exactly like in real life, you'll never get it 100% right. But maybe 98% could do?
Keep it up :)