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....
But if thou do that which is evil, be afraid; for he beareth not the sword in vain: for he is the minister of God, a revenger to execute wrath upon him that doeth evil.