Next Construction Question?

I'm building a wishing well, its in a 48 inch diameter circle. The walls are 2 x 4's cut 7 7/8 inches long and they lay on the perimeter of the circle 10 high. The plans call for a 2 x 6 cap at the top of the wall. The cap is angled so that all 19 pieces fit snug together when the cap is done. The 2 x 4 walls are not angled, probably a good idea as would give good drainage when it rains. What angle do I cut the 2 x 6's to make a snug fit on the cap with 19 pieces cut at 7 7/8 inches. P.s. the boards can be longer or shorter to make the snug cap.