I've built more than a few permanent stands, and I liked to put them in red maples, usually a triple off the same stump. I started by attaching a 2x4 between 2 of the trunks (trunk 1 and trunk 2 - the ones oriented toward where I wanted to be looking most) at the height I want the stand to be. Get this piece level. It can be left in, or later removed. I used it to make sure the ends of the floor supports that rest upon it are also level. Then install 2, 2x4's or 2x6's that are the floor supports. One end of each rests upon the first 2x4, nail them to trunk 1 and 2. Then nail the other ends to trunk 3, making sure they are level with each other. Now you have the 2 floor supports that run between trunk 1 and 3, and trunk 2 and 3 installed and level. Add your floor. I always liked 2x6, spaced about a half-inch apart. Your seat can be any number of things, but I liked a strap-on tree seat. That way I could attach it to any of the 3 trunks. Add a 2x4 rail if you like. To get into the stand you can use screw-in or tie-on tree steps, or an old aluminum ladder.

I always used spikes to attach to the tree (can't recall the size) and 16d nails to attach the floor to the floor supports. Never had a problem. I never added sides or a roof. If possible, I put these near a hemlock tree or other evergreen that would offer some cover, but they did work out in the open too. Once installed, I sometimes used a spray can of grey or brown auto primer to dull the new wood.