Well that makes sense! I take it you don't use these then?
I think I may have one lying around. I made a single chisel out of an xActo knife blade. Steady hands, a good eye, simple tools, and pickiness will usually do a better job than the "perfect" job done by simple gizmos like that.
I scored a powered slotting tool at an RC club raffle, which, if you're not using wood that's too hard, does a job that's at least close enough to be repairable.