The arm protectors have two functions: one is to protect the arm from string slaps, and the other is to keep clothes out of the way of the string. The smooth plastic also may mean that if the string goes on the protector, it won't distort the shot extremely (though it will distort it significantly).
These are usually made of leather, but I don't have any scrap leather.
Step 1: Cutting acrylic
You can cut acrylic sheet by scoring deeply (a couple of passes may be needed) with box cutters, and then breaking (while wearing safety goggles, of course). Then you can smooth the corners and the edges with sandpaper, a file or (in my case) a sanding drum on a rotary tool or drill. As always, when sanding plastic (and other things) wear breathing protection.