Boxes are good. Cheap, easy to find and with a little paint will look ok. Or you can use Foamboard, as Craig suggested. The final Option is using hobby wood from Michaels,...but it is rather expensive. Best bet.....Old boxes and a new can of Spray paint. Gray Primer is the Dio's best friend! I usually keep about 4 cans in the garage! At $.97 a piece at Walmart, you can't go wrong!