Depending on the age of the case, it may be a flat key too. I have 4 machines here that use a flat key instead of a square one. These should be even easier to make, in theory, should be able to do it with flat stock and a file.
That said, both of the keys I have are different, one has a "flange" that holds the key in the lock unless it's locked, and one is just straight. The flanged one is circa 1936, the straight one is 1912. I can post pics if needed. (They're my 2 latest acquisitions from Calgary this weekend)