I would just buy something like this, as it already comes with Alien Bee speedring and much cheaper to boot:http://www.amazon.com/Fotodiox-10SBXAB2436EZ-Solutions-24-Inchx36-Inch-Speedring/dp/B003EWKPEC/ref=pd_sim_p_10It can even be used as a pseudo-beauty dish with white cover removed and center reflector. I have the 36" octagon version with same features, and it's been great.
buy the speedring that PCB sells on their website. the connection to modifiers is typically universal (some rare instances where a modifier wont fit that well) so you have to buy the speedring that will mount to whatever light you are using.