Remember the old Sony Ericsson MCA-25? Once upon a time it was a camera attached to Sony Ericsson T68/T300/T310. I recently found my old MCA-25, which has a small circular viewfinder. The circular viewfinder has the 'fisheye' effect, so I decided to take it apart and use it as a camera 'attachment'.
The idea came from 'jelly-lens' that I just bought. To be honest, the image captured using MCA-25 viewfinder is much superior than those captured using jelly-lens. What you need is just small dots of blu tack.