Good Afternoon, Ian,
Ordinarily, sheet film is sent to a lab in its original box or another one of the same size. Regardless of the box used, be sure to label as to film type (E-6 or C-41); proper labeling will save a lab technician the bother of having to read the notch code, although that will probably be quickly double-checked anyway. I have often sent color film in boxes which originally had B & W film in them; with correct labeling, that has never been a problem. 1 1/2 or 2 inch masking tape wrapped around the box gives an ample amount of space for writing name, film type, number of sheets inside, any special instructions, etc. A Sharpie pen writes well on the tape.
Konical