Commercially made zero-clearance inserts cost about $20. I make my own for less than $3 from 3/8-in.-thick polyethylene cutting boards, which are flat and rigid. A 14-in. x 17-in. cutting board ($10 at a discount department store) yields four inserts. After rough-sawing the blank on my bandsaw, I attach the saw's insert with double-faced tape. Then I rout the blank to final size using a flush-trim bit. I drill 11/64-in. pilot holes and install the adjustment screws, which are 3/8-in.-long #10-24 Allen-head set screws (about 40 cents each at a hardware store). Raising the blade through the polyethylene to cut the zero-clearance slot leaves plastic curls around the edge. They're easily removed by filing or sanding.