Free: Contests & Raffles.
Probably OK as long as you don't cut anything live to do it. Just use dead slash and debris. You will have to understand that some other hunter may find it and be using it when you get there during hunting season. First come first serve. You have no ownership rights.
back when I was a kid and I hunted ducks liked they owed me money; I built a blind on public ground out of drift wood. It got to be such a popular spot I would have to get there at two in the morning to procure it. one morning I got there a little late and saw a guy just leaving his truck with decoys on his back, he saw me and I SH!t you not, he started sprinting to get to this blind first.
I know a guy that makes nice blinds in place for the Elmer Fuds to find.... They are on the way to good spots, or in a place where the birds don't like to go and force them to the hot spot... If you build it they will come... the best blinds i've had were carved out of blackberry bushes with a pair of hand snipper.