You can buy it in liquid form, but it's not what you want. Go to your local hardware store, and pick up some sheets of lexan of the proper size. Trying to mold a liquid into the right shape is quite difficult, and the lexan will be clearer, stronger, etc. Cut it slowly with a jigsaw or table saw, being sure not to heat it up, drill holes for the hinges, latch, etc, then just bolt the hardware one.
--Bushytails