I guess you thought of this already Moonchild; but I have a small Maton with something like those dimensions--a bit more depth-- and I just use a classical case. I take one of those soft furniture polishing rags (untreated) and wash it a few times. Then I tuck it in at the bottom (tailpiece end) of the case to make it a little more snug. The rag comes in handy for wiping down the strings (and the instrument itself when I recently had a hay-fever attack while playing; but that doesn't bear mentioning!)

Murray