Completely unrelated to your question.... but use to do a lot of crow hunting (I know sounds stupid, but it's a riot) Anyway.... would pile up big brush piles with tractor and then just go in there with chain saw and "carve" out places to sit. Worked well. Man that was some fun shooting.
Like other said.... any brush, sticks, stumps, whatever. Get some heavy duty black zip ties as they come in very handy to get things to stay put. Rebar tie wire works great as well. Tie wire works great for treestands too.... you can just pull branches out of the way and tie them off instead of cutting.... that works really really well if hunting out of a everygreen tree (my favorite).