[lunarhighway]

here another thing that just crossed my mind: get a plastic bag, maybe a garbage bag or something, put the carboard inside, fold the bag as closely around the card as possible, tape it tightly together ... even an external block wouldn't look to bad if done carefully.

any kind of plastic would work by itself too so perhaps some squarish PE containers from distilled water or soap, could be cut up to from some rectangles that give you the right surface... you could weld the edges together, as glue (and paint won't bond to PE) or perhaps staple them together