I agree with UncleNorby about using pallets. If you're going to go through the time and trouble of building a nice stand I recommend you invest in treated lumber.

I built a 16 ft. free standing one out of treated 2x4s, metal electrical conduit, and angle iron from an old bed frame for about $60. It's kind of like a giant baby's high chair and real nice for an all day sit. Also, it can be positioned anywhere you want and is not dependent on a tree location.



The unit can be assembled/disassembled on the ground in the field, then raised into a standing position. One person can stand it up, but it's a lot easier with two. It's very stable in the upright position. The front ladder, rear frame, and seat unit, and cross braces are attached with 1/4" galvanized bolts.