I usually make mine from 3/8" Baltic Birch ply, as do others here. I usually make a batch from one template, install a dado stack in the saw and remove most of the birch under the future slot. You will find that most 10" table saw blades on most saws will not retract the full thickness of a 3/8" plate. I also like drill a 1" finger hole in each plate blank and chamfer their edges.
I use four each, #4 flat head wood screws for leveling, installed from the bottom of the plate, either Phillips or flat blade. If you make them from phenolic or other plastic, they might last longer, but they also dull the blades and bits used to shape the insert and cut the slot and the dado stack. I like to write the angle of the slot on the top and bottom of each plate in large black letters the first time I cut the slot. Make sure the angle is written so that it is right side up when you hang the plate from the extraction hole.
If you want to be really picky, stack them in order by angle on the peg. Some organization is even more important for your dado stack zero clearance inserts, since stack width, bevel angle and depth of cut can all be important for these plates. plates