The best material is probably 3/8" or 1/2" Baltic Birch plywood.

Use your existing throat plate with double side tape as a template and a pattern router bit to cut the exact shape. If you are expecting to make more than one (duh) ZC insert, make a permanent template out of any suitable material. (I usually use 1/4" melamine for these types of templates as it is easy to remove the DS tape.)

Use a Forstner bit to drill depressions in the underside of the ZC to match the fingers in the throat plate opening of the table saw. I use 4x40 machine screws as height adjustment screws. (Drill a small pilot hole and force the screw to cut its own threads.) I use the machine screws only because I have a whole coffee can filled with 3/8" length. (Don't ask)

To make life easy when cutting the ZC kerf use this trick. Measure the distance from edge of the throat plate to the side of the the tooth of the blade and subtract 1/32" from that measurement. On your router table, with a 1/4" router bit in place, set the clearance between the bit and the fence to the above measurement. Use this set up to route a groove about 5" long and about 1/2 the thickness of the Baltic Birch deep approximately in the middle of the ZC insert. This groove gives enough clearance to allow the blade to spin freely in the fully down position.

Adjust the height of the ZC insert to be level with the table top using the machine screws.

Using the original throat plate as a guide, put flat head screws into the side of the ZC insert. Use these screws to adjust the ZC insert to be wiggle free.

There is usually a small lip on the out feed end of the throat plate. The ZC needs to hook on that small lip. Put a small screw into the out feed end of the ZC insert. I usually use a flat head screw and file the top side of the screw head to fit nicely into the lip.

When cutting the kerf in the ZC insert, I like to clamp a 2x4 on the table saw, front to back. Then raise the blade as high as desired.

If you don't use the router table to cut the 1/4" groove above there is another trick. Clamp the 2x4 at the user side of the table. Use some DS tape to hold the ZC to the 2x4. Raise the out feed side of the 2x4 about 1-1/2" to clear the blade and prop in place. Start the saw and from the out feed side gently lower the 2x4 so that the blade starts the ZC kerf cut. Clamp the out feed side of the 2x4 and raise the blade as high as desired. This process needs to be done before installing the locking screw in the outfeed end of the ZC insert.

You may need to install a "friction" screw in the infeed end of the ZC insert. This screw just gives the ZC insert a small bit of friction to keep the ZC in place.

Drill a finger hole as appropriate to allow for easy removal of the ZC insert.