I saw several around-the-tree seats on a Google search, and I decided to make a half round seat for our maple tree. With a compound miter cutoff saw, cordless drill and measuring tools I set out to make a bench.
The wood I used came from boxes and crates we got at work with machinery shipped in them. First, I took a small chair and angled the back 2x4 to the same pitch and trimmed off the bottom, then decided on a height and cut it to length.
Then I cut a seat support and front leg board, and secured them with small triangles, using galvanized deck screws, to form an"h". I made four of these "h" legs and stood them by the tree to determine how far apart to make the seat sections. Since I was going half way around the tree, or 180 degrees, and using three sections, I figured the angle at each section would be 60 degrees at the intersection.
Next, I cut the front and back cross supports with the ends trimmed at 30 degrees (combined to be 60 degrees) and attached them to the four sections. The back supports were a bit tricky. I had to miter cut them at 30 degrees, and 7.5 degrees to match the angle of the slanted back. Those I attached with deck screws as well. (In fact, my entire project was secured with deck screws and some wood glue.)
The back boards came next. I used 1x4 boards just a little longer than the back support board and cut 45 degree corners on the tops. By drawing a line on my saw back stop, I didn't need to measure each board; just hold it to the mark and cut. I spaced the back boards out and screwed them in place. Next, I measured out the seat boards, and cut them to 30 degrees and secured them to the "h" leg supports.
For added strength, I cut triangles and mounted them to the leg supports, and cross member and both screwed and glued them in place. I decided on an arm rest on each end of the bench, so I added an upright 2x4 at the front, a small triangle on the back, and a horizontal board for the rest. I screwed & glued the triangle and arm rests in place. A little sanding, a coat of exterior primer, and two coats of my wife's choice in color, and there you have it: a tree bench.