I based my ringflash on a DIY design on the discussion threads at Strobist.
It is basically a 99p Wilkinson's 12" diameter plastic salad bowl with the interior spray painted white and the exterior sprayed black; a hole at the base for the lens; and a hole in the side near the front edge for the head of my Canon 540 flash.
There is an inner made from two plastic tumblers glued over the lens hole, inside sprayed black, outside sprayed white.
Between this inner and the salad bowl, I attached diffusion material layered in thickness 3-2-1 moving away from the flash to try and even out the intensity of the light.
I used plastic because I haven't the tools or skills to cut metal and it is lighter.
I attached a meccano-like frame (various bits from the local DIY store screwed together) to the bowl to firmly seat the flash and provide a hand hold so I can hold the ringflash with one hand and my camera in the other.
I fire the flash with an ebay poverty wireless trigger.
I use my 50mm 1.8 prime lens as I'm shooting close to the model and there is no way I could use the zoom on a zoom lens with it stuck in the ringflash.
I shoot on ISO200 and usually set the flash to 1/64 power and shoot around f3.2. Saves battery power and blurs out the background nicely.
The circular white "vignette"

that intrudes on some of the pictures is totally accidental and due to the "play", because the camera and ringflash are not on one mount, between the lens and the inner of the ringflash but I think it sometimes adds a nice "feel".
There are some examples in my Gallery
Paul