Opie, I would shy away from plexi simply because I don't think it could take the heat.
'Real' (read: stuff you can buy lol) reflectors use tempered glass.

If you are already running an air-cooled reflector, you might try jacking up your air flow rate. When I was getting mine set up, I ended up having to use 2 inline fans to move air because the exhaust duct was 25 feet long and had a couple sharp bends. Just a thought.

Where is the air going when it leaves your hood? Are you exhausting it within the grow area, outside the grow area, outside the BUILDING...???