This is how I affix my rear light because I strongly believe rear lights are best fitted so their beam shines (more or less) horizontally, into car-drivers' eyes rather than cyclists' eyes.
It uses a P-Clip (with the rubber insert), a standard Cateye bracket and a plastic block (I had loads left over from building some flat-pack furniture). The blocks are pre-drilled, strong and light. I wrapped it in black tape.
It's rock-solid but can be tweaked up/down or left/right easily. Without the plastic block the light hits the seat stay before I can get it horizontal, although this is a Cateye 600 issue that wouldn't affect other lights I imagine.
Some Star-washers, to dig into the plastic block, may make it even more solid.
I may turn the P-Clip over to get the light more in line with the seat stay.
Now I'm happy with the way this has worked I think I may stack another light on the same seat stay. (I just like having 2 rear lights, flashing on different patterns).
The close-up photos make the setup appear more obtrusive than it is in reality.