It is a centrifugal fan, so the casing is an integral part of the fan.
That said, the size is not all that critical. Any round or semi-square plastic tub which will fit over the end will do the job fine. Just fit the tub over the end, and cut a 1"-2" hole in the centre of the base. Put a grill over the hole for safety, and Bob's your father's brother…
(whoops – SNAP! What he said, above….)
Edited By Dodgy Geezer on 06/10/2016 22:39:22