One thing I have learned, especially with my saw, when building these boards is I rum the edges through the jointer first. Until I can figure out why my table saw plate sits lower than the table saw deck.
One thing you can do is use a hot glue gun to make custom sized spacers.
1. Remove the insert and rub wax on the contact points where it touches the tabs.
2. Put melted hot glue on the tabs.
3. Let glue set up for about 5-10 seconds.
4. Gently install insert. Be careful to NOT push it below the surface of the table.
4. Using a straight edge to span across the entire distance of the insert and onto both sides of the table, gently push insert lower. Make sure you push a little down on the front, then back. Go back & forth until level.
Hopefully this makes sense. It was a Woodsmith Shop eTip a few months ago, but I can't seem to find it right now.