I made a zero clearance insert for my saw. It came out real nice. I have not yet cut the blade slot. What is the best way to do that and how wide should the slot be? I was going to block the insert in with a couple heavy magnets and raise the running saw blade into the insert.
A zero clearance insert (sometimes referred to as a ZCI) offers minimal clearance around the blade while cutting and provides support for the piece being cut. It provides for a safer operation and a cleaner cut. It does help prevent debris from being jammed between the blade and the insert. Only an absolute fool will stick his fingers there to clear it out while the blade is running. Not a common "accident", thank goodness.
While you have the one you made (call it a template, or sample), make a few. Cut your stock close to the line, and use either a band saw, scroll saw, or a jig saw, and cut close to the line. Then, double side tape it or hot glue it to your template, and rout the new one with a flush trim bit with a bearing.
Place the template in the opening, and if it needs leveling, you could insert small screws in the bottom side to raise and lower it. With many saws, ½" material, like plywood works very well. Your particular saw may require a thinner, or thicker piece.
Crank the blade all the way down and place the template in the opening. Your blade may not retract enough to clear the bottom surface. If that's the case use a blade from a hand held circular saw (7¼") to start the kerf. If you are using primarily 10" blades with an eight inch kerf, find a circular saw blade with the same kerf.
You can move the fence over the top of the template (but make sure the blade won't hit it when raised). Or, you can clamp a board or plywood to the table to hold down the template. Turn on the saw, and slowly raise the blade through the template, and raise it all the way up. You're done.