I suspect that one of the hobby die cutters is a better place to look than anywhere. They come in all kinds of shapes, but nothing that I've ever been able to find that matches. They're simple blade inserts, but making one at home might be tough without some expensive cutting tools for metals.

However, if film does begin to transition to the hobby market, then you can bet those folks would be willing to run the numbers to see if they could make a profit at it. This stuff is usually in the scrap booking section at places like AC Moore and Michaels in the US. Not sure what retail outlets would be in other locations, but they've got to have stuff like this.

If you could get one tool handle with two inserts, one to make the notched end for the cassette spool, and the second insert to make the tongue for the leader you would be in business.

Film is actually thin enough that the dies could be made from hard plastic, but a metal one would last better.