I think expanded metal could be used in addition to the thick plexiglass to stop larger pieces.
Plexiglass is NEVER an option with mulching equipment. Just like it has "glass" in its name, it will break into sharp pieces and can cause severe injuries, or worse.
Lexan (Polycarbonate) is totally different. You can slam it with sledge hammers and never break it. Lexan with a hard coating is the standard for any mulching equipment. It is extremely expensive so treat it with care and never use any chemical cleaners on it. The only chemical that I do use on it is Novus plastic cleaner with a 3 stage process that will usually pull out minor scratches.
Metal guarding in front of the Lexan will help it last longer and prevent some scratches but don't let it touch and keep it wider at the bottom so debris can self flush. I usually use a 2"x2" to 4"x4" cage on most of my equipment. Expanded metal is difficult to see through and has never been a viable option for anything I have used. Goat panels from TSC built up my last set of side guards. They are not very tough but they do deflect a lot of the side hits on the doors and reduce the accumulation of deep scratches. My front guards usually use thicker rods that don't deflect easily. Cedar trees are one of the worst for jumping into the guards front or side and they scratch it up fast. Most any tree can get you killed if you don't pay attention.
The only thing that I still run glass on without anything more that a heavy screen cage is my excavators, and that is still risky. When mulching, I always wear a hard hat and safety glasses in these machines because chips/chunks fly into the skylight and occasionally, I break the front glass. This has only happened in extremely rocky ground (once) and when a rope with a hook at the end (once) was wound up into the cutterhead and flung the hook through the cage and right into my cab glass as the rope whipped the corner and broke the hook off. In the past, I've seen 6 lb hammers fly off of my cutterhead to who knows where at near full speed. I'm damn careful where my head is pointed and I don't use flail heads anymore.