I propose using only one piece of wood, cut to the circumference of
your circle. (I'm sure _someone, somewhere_ must sell 30-ft. railroad
ties!) You can just use a 90-degree angle on both ends, and then run
it through your railroad tie bender (available from your local home
improvement store for a mere $499.99), and there you have it!


Seriously, I wouldn't be too concerned about an exact angle because the
ties will probably shift over time and something will inevitably grow
up through the cracks anyway! Not exactly like building a picture
frame.
OK, now everyone can flame me for my "rustic" approach.
Gary