Trying to bend a long, flexible strip and trace an arc at the same time can be tricky. To make it easier, I made an adjustable arc that can be locked into any curved shape I want. The arc is made from a strip of 6mm pegboard (about 600mm long), a nylon string and a cord lock that I bought at a fabric store.
Easy Assembly. First, cut the nylon string to twice the length of the pegboard strip and tie it to one end (main photo). Next, slip one end of the string through the cord lock and then loop that same end through two holes in the opposite end of the strip. Finish up by feeding the string back through the cord lock (inset photo).
Adjust to size. To change the size of the arc, simply move the looped end of the string into a different set of holes in the pegboard strip and adjust the cord lock.