Bob Mill wrote:

That's kinda was I was thinking, but here's a dumb question. Since
they old insert clipped into the table, but plywood insert isn't going
to be able to do that. How do you guys secure your homemade insert
into the opening?

snip

I don't; however, after placing a blank plate in the opening, move the
fence just over the edge of the insert to hold it down while the blade
is lifted up to cut thru the blank insert.

After that, it is unnecessary to secure the plate.

Gravity does the job.

Lew