Picture frame with string

Thanks to Ruben's comment I was able to make the string for my picture frame.
(I can't seem to post comments atm, blogger's behaving really strangely, so I can't reply...)

1st try with Ruben's advice:

 2nd Try Successful:
(I decided to see if the string size depended on how big the polygon was when when you extrude it, and I was right ^o^)