I don't think its made for speedlights but it seems like some people are using it that way..
what I found in the reviews about this octobox on BH.
I've been looking to add a softbox to my lighting setup but wanted to use my Nikon Speedlights as my light source. This softbox came with a "generic" mounting ring which made it easy to pair with a bracket for holding my Speedlights. Setup was a bit slow as their were no instructions, but I went to the website and found what I needed. Inside baffle spreads the light nice and even. Great product, great value.
This octobox probably would have got a full 5 stars if there would have been any instructions included. I managed to put it together in about 11 minutes and discovered I put the ring in backwards. I am rating it based on the price so it is a good product with a good build quality. I planned to use it with my 580ex ii, but that will require some additional hardware that I ordered from Cheetahstand