Start by measuring the thickness of the table insert. You want to match that as closely as possible.

If it is 1/8th inch thick you can use masonite, or tempered hardboard. For other thicknesses, check out the window glazing area of yor local home supply store. they frequently sell plexiglas, lucite, or some other clear acrylic plastic. This comes in various thicknesses (pick the nearest one) and frequently they sell small pieces (under 1 square foot) for reasonable prices.

If your material is a bit thin you can "shim" the thing by placing small, closely spaced drops of hot melt glue around the edge on the underside. The glue drops should sit on the lip that holds the insert.

Of course you can forget about a zero clearance plate completely if you just cover the table of your saw with a piece of 5/8 to 3/4 inch plywood. This effectively gives you a totally flat, new, table top onto which you can nail, bolt or screw any holddown device, fence, or jig that you can imagine. If you have a router, you can add a slot for those crosscut tools that need to have one. With the price of a 2x4 foot handi-panel of 3/4" plywood being about $6, and a sheet of plexiglass being about $4, there is not a lot of difference in price. But, if you are thinking of ripping strips, when you mount that 2x4 piece of plywood onto your tablesaw, in line with the blade, you get a really nice, long, infeed side for supporting those boards you are ripping -- and you can easily make a nice long fence to guide them, too. In fact, you can make a "split" fence form two pieces of stock. The first maintains your strip thickness all the way up the the far edge of the sawblade, and the second moves back, away from the blade, at least 1/8th inch, to give additonal clearance for the strip so it doesn't spring back against the blade and bind. Since your "new" table top is wood, a simple "splitter" or kerf follower would be a 8d finishing nail driven into the plywood in line with the blade, and about a foot behind it.

One more advantage to the 2x4 plywood panel: If you are going to do bead and cove edges on strips ou won't need a router table. You can mount your router underneath the plywood and it will be at the same work height as your table saw. Depending on where the router is actually mounted, you may be able to use the same infeed fence, too.

You make your new slot in whichever you chose -- complete top or blade insert -- in exactly the same way.

Hope this helps

PGJ