Make a zero clearance table saw insert that is both accurate and cheap.
Step 1: Stock
Start with a flat piece of hardwood. I chose maple.
Step 2: Width
Cut the stock to the exact width of the insert opening.
Step 3: Compass
Use a compass to find the arc of the insert opening and trace it onto the wood.
Step 4: Sand
Shape the wood with a disc sander. Fine tune the edge with a sanding block to get a snug fit.
Step 5: Jointer
If the stock rises above the surface of the table, remove some wood so that it lies just below the surface of the table.
Step 6: Tap
Drill holes in the four corners of the insert corresponding to the hold shelves on the inside of the insert opening. Tap the holes and insert headless hex screws. Adjust the screws so that the insert lies flush with the table top.
Step 7: Cut
The 10" blade on my table saw did not lower enough to to allow the insert to be completely inserted so I attached a 7 1/4" blade. Make sure the kerf is the same as the 10" blade. With the insert in place, turn on the motor and slowly raise the blade, cutting a slot for the blade.
Step 8: Big 10"
Remove the smaller blade and attach the 10" blade. Repeat the previous step.
Step 9: Finish
Coat with Danish Oil.
Step 10: Finished
At this point the zero clearance insert is complete. The following steps are to include a splitter or rivining knife.
Step 11: Addition
Cut a small line in the bottom of the insert to see the blade line. Drill holes to accomodate your riving knife or splitter. File the hole smooth.
Step 12: Fit
adjust the insert for fit.