It's tempered hardboard (Masonite). I want something stable and about 1/4" thick. MDF will also work. You can download the procedure with a materials list via the link in the description.
That's the Lie-Nielsen No. 9 Iron Miter Plane, based on the Stanley No. 9. L-N no longer makes it, although I've heard rumors that they occasionally do a run of them.
The wedge was really helpful at the school where I taught. We had a bunch of shooting boards that students used in various classes. As newbies, they would often rock the shooting plane and cut into the fence (wedge), which then created tearout on future projects. It was easy for me to fix the issue by taking a few plane swipes on the wedge and re-trimming it. Prior to my taking over the handtool class (in about 2004), the previous instructor used the wedge design, but the wedges were tapered in the other direction. That wasn't helpful! That's how it started. I like that the wedge can be fixed easily if necessary. Also, once the dado for the wedge is set correctly, the angle is no longer adjustable. (Things that are adjustable tend to get out of adjustment.) I realize the wedge design has some disadvantages, but it works for me.
