Al, the cost of making up a IXMOO style cassette is prohibitive, unless it was made in huge quantities (10's of thousands). The die's alone would cost in excess of $60 000 each (and you need two - one outer and one inner shell).
There are still IXMOO's out there, though they are getting difficult to find. The center spool is complex and, as you say, can be replaced with common plastic spools and tape.
I also use the Nikon cassette (enough to do 200ft of film) - and it is a less complex design - and easier to load - but also more fragile. The locking spring on the top is prone to getting tired and that makes the cassette virtually unusable.
The trick is to accumulate about 18-19 cassettes and then load 100ft rolls into them and just shoot away. The cost of the IXMOO's, though initially high, will be recouped after a while.
I did spend a fair bit of time collecting my current stash of them. Some traded, some given to me and some bought. At the moment I have enough to load about 1000 ft of film (180) and that will see me through 3 month of shooting.
