If you go to your local hobby shop you can find some styrene plastic sheets that are molded to look like brick, siding, etc. You'd have to do painting and detailing on your own, but you could probably find some stuff like that if it's a good shop.

 

If it is the side walls of the "box" I'd definitely go something more rigid than cardboard. I generally use marine plywood as there is no possibilty of warping or getting bent under pressure. Nothing worse than trying to move a dio and you need kid gloves to handle it. Build it sturdy from the start.
I'm sure John (Reid) can give you some expert advise as well.
:cheers

 

Thank you guys... :bowing: I really appreciate it and will take the advice. :3tens: