I love fisheye photography and I own a Sony RX100. Unfortunately there are no cheap fisheye lenses for this camera (only some expensive ones for underwater photography).
So here is how to make one.
1) buy an Opteka 43mm ultra-fisheye lens. These cost around £60. Not super-cheap but not that expensive either.
1) Take a toilet paper roll cardboard.
For absolutely no reason, the toilet roll cardboard fits perfectly around the objective. A case of random perfect fit. There probably is a Japanese word for that. See this flush cut (you are not going to make a flush cut for the mount, this is just to show the fit).
3) cut the cardboard around 43 mm in length. You have to adjust manually taking a few pictures and seeing what the result is.
4) Once you find a trim that gets you a decent picture quality, wrap the cardboard with tape, so a) it sticks to the lens b) it doesn’t lose paper dust and c) can stand a few drops of water.
Done! The quality is medium-low, but with some photoshopping one can get some funky pics