I used TechShop San Jose to do the work.
scrap plywood (for frame rails)
2x4 (about 8')
13-14 redwood fence boards (these were about 7.5" wide)
fasteners (3" deck screws, small gauge brad nails)
Step 1: Simulate design on Autodesk Inventor
Step 2: Cut top and bottom rails on Shopbot
I used VCarve to create the tool paths to cut the arcs out. It was to be about 6 feet wide, so I drew a large circle (about 94" radius) and then trimmed it to be 72" wide. The shopbot CNC router made much cleaner cuts than I can do by hand with a jigsaw.
If you don't have access to a CNC router, you could draw a pair of arcs using a piece of string and pencil. The arcs I used were about 90 and 94" radius for the top piece.