Part of me agrees that you should just check out an aluminum ladder stand, but if you're dead set on building one, this:
Originally Posted by
VA5326
Buy yourself a book on building tree houses for kids. My buddies have built a few with modified plans from a tree house book. Wish I knew the name.
... seems like your best bet.
Although for all the trouble involved, you may just want to build an elevated blind with pressure treated wood and shingled roof. That way you'll have something to keep you out of the weather, and won't be as prone to rot as something nailed to a tree.