Worst case is to use a multi-blade hacksaw to rough it in and then clean up with a small file. I didn't like it either when I had to do it. But, in aluminium, it isn't hard. The best bet is to acquire the broach. A shaper, if you want one. But it should be as a generic tool, not just cutting keyways.
It's all about time and money. The only advantage to using hacksaw blades and a file is it's available "right now!" and it's cheap. But otherwise, a PITA.
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