I put it in a small cardboard box that could just fit a ZL envelope, because I would want to take good care of the envelopes sent by You-tai too.
When opening the envelope, use scissors to carefully cut a small slit along one side of the envelope. Try to open the envelope while keeping it intact.
However, this would make it quite difficult to review and search through, and some other stations use envelopes that are larger than ZL, which wouldn't fit in the box.