There are many good optical fiber-based DIY ringflash adapters. For examples see the following links:
However, I found a bit their installation a bit complicated. To make such adapter more user-friendly I built the whole concept around a tube that can be slide around the lens. Below I provide a tutorial on how I built my ringflash adapter for sony nex-5. You can follow the same principle to build your own for any camera type.
I bought a plastic waterpipe in the local hardware store. I used 75 mm diameter tube, but you should consider that there is an effective subject to flash (here adapter) distance for ringflash work. It's approximately between the half of the diameter and twice the diameter of the ring. So the optimum for my adapter is between 4 and 15 cm. For 100 mm macro lens with longer subject to lens distance you have to use larger diameter tubes.
I cut a 10 cm piece with fretsaw:
I glued the three velcro stripes together and fitted the fibers through the plastic mount and glued it (you have to use a special glue not dissolving the fibers. I used a polystyrol glue from pattex). Later on I trimmed back the overhangs and polished them.
The flash adapter works well but the sony nex flash is really weak. It has a guide number of 7, but still at ISO400 I can use f16 most of time (depending on the distance and the colour of the subject).
For results, see my flickr page:
If you have any question or suggestions, don' hesitate to contact me. You can also send me picures of your own version, I will post it on my blog.
Have a good tinkering.