I had 5mmx4mm neodymium magnets on hand so I made a hole in the design so I can super glue the magnet. I don't think you need neodymium magnets, it's just what I have on hand for other projects. You can probably also use other magnets. It may not even be required, I didn't even test.

Also after printing there might be small blobs around the area where you insert the blade. Make sure all of that is smooth and the blade can be seated completely into lever arm otherwise the lever will not spring back.