I've been working with Google Sketch Up to draw some of my ideas. The first being the storage box that holds the 4 modules, below is the box, that will accommodate 4 modules, although there is no back or front drawn the idea is to have both, the ledges would be metal making the modules slide into the case easily.
The below photo is the Phillip Island and District Railway Modellers Module carrying case, thanks to Judy and Jack for supplying the photo!
Cheers
Mark