Hi Burr...

Could you share step by step on how do you create the walls ?
You can use this free program to create a video tutorial : http://camstudio.org/

Update : OK, I can figure out your idea, but moi take more time to calculate shell. The best methode is extrude :)
My Laptop have 4 Gb of RAM, Core 2 Duo and NVIDIA Quadro FX 1600