Planet bike's tail light bracket works with Topeak and Blackburn racks as I have installed it on both of these racks.
Easy to install and hardware is stainless so it doesn't rust.
Works with other brands as well. I have old Trek and Specialized lights that slip in securely.
However, having read forums about bicycle lighting, it is possible going over bumps that your light could be dislodged and fall off or even apart.
To solve this problem I use a thick rubber band to secure the light to the bracket and so far I have not lost a light.