You planning on building it so you can fold it down for transport? Otherwise you may have to "touch up" the vegetation from time to time.
I would use some canvas type material or camo cloth sprayed with water repellant....to help keep the rain/snow off.... maybe under the burlap as a windbreaker too.
I'd probably use some of that synthetic grass type material as well... lighter weight and more durable.... you can get it from Macks and Cabelas I know....
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