Installed in saw; zero clearance inserts can be made out of wood. This is one I have been using for years. It is made out of 1/4 in plywood and hardboard. The insert is cut at a 45 degree angle like a dovetail. See attached pictures with captions. The bracing on the bottom is half-inch...
I put a dado in the edge and epoxied in two magnets. It doesn't vibrate at all. Pretty proud of myself because I came up with that one all by myself :)
I also glued the block under the far side of the insert so that it wont' fly out...probably overkill.
I got this 24x40 piece of corian from the Habitat re-use center for $7. It made several really nice zero clearance inserts. Notice the holes for set screws. Who would have thought that you could tap corain and it would hold!