The plastic is styrene, available from Walters (among others):
https://www.walthers.com/
The .040" and .080" thick sheets work well for yoke covers.
It is very easy to work with. Simply score the sheet, and when bent will snap at the line. Pieces can be fused (melted) together with styrene glue. I have repaired a number of yoke covers and even the entire yoke body, after being trained by Bob G. Try it, it is not hard or expensive. :-)