Found an image on google kind of explaining what I'm thinking of.
Not quite a flat plate but something like the below that encloses the blade, you could take it to the next level and have the sides lift like the overhead guard does. My logic is to make an area around the blade that most of the air movement is caused by the spinning blade. This will hopefully push the dust forwards towards the end of the guard where it'll get picked up by the overhead guard extraction point.
Or will the dust just hit the top of the secondary guard and just bounce all over the place
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