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I've done this before with HDPE instead of wood. In a really humid climate it seems to work better. On my older sliding compound miter saw that doesn't have a replaceable insert, just an open trough. This saw needed a different approach for zero clearance. Since there was no place to bolt an insert I ended up filling the gap with body filler. It can be refilled as it wears or if you widen the kerf cutting bevels. However you end up creating the zero clearance insert the results will always be worth the effort.
I keep coming back and looking at your finished chairs, they're very nice. The first thing that came to my mind when I read you were using oak, was "How heavy are they going to be?" Have you weighed them, or do you have an estimate of the weight of the finished chair? I think I might build a pair out of salt Cypress, if I can find any.