well, would you believe that i had a normal sized cake box just lying around... so what i did was copy the shape on a much smaller scale and cut that out of the cake box cardboard
the template looked something like this...
just make sure that the base and lid are squares of the same size and that they are bigger than the diameter of the cake
the rectangles form the side flaps.
the long sides are the same size as the squares and the short sides should be a little longer than the height of the cake
when you assemble the box the glue goes on the flaps and put the flaps on the inside of the box so the outside looks neat
i hope that makes sense