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You can make one out of plexi but be aware that unless you add supports or ribs to the plexi, it will bend over time. Another option is to go to your local glass store ( not HD or Lowe's ) and have them cut one for you to size. I used to go to mine and ask what they had for scrap glass ( which they often gave me for free) and if it was big enough, I paid them to cut it to size. Don't get thin cheap glass as it can shatter on you.