How to create a level with curved lines with cocos2d + Box2d on the iphone?

Question

Hello,

I'd like to create a game that has levels such as this: http://img169.imageshack.us/img169/7294/picdq.png

The Player moves "flies" through the level and mustn't collide with the walls. How can I create such levels?

I found that piece of software: http://www.sapusmedia.com/levelsvg/

It's not that cheap, so I wonder whether there is another way to create such a level as shown in the picture above...?