head on over to a local glass shop, or give an auto glass shop a call and see what they would recommend for you, they'll probabaly recommend another shop, but at least you'll know where to start from.
You can always use Lexan or similar product, lexan can be coated with a special scratch resistant surface that makes it quite durable.
But, your best bet is your local glass shop, they can cut the glass to the exact size you need. oh yah, you WANT saftey glass, like they use in car windows. It is very hard to break when hit head on so you don't have to worry too much about it shattering. Just DON'T hit it edge on.... it will shatter easier that way.