Rear mounted hydration system for Selle SMP saddle

I just got a Selle SMP Evolution for my tri bike and I am looking for a rear mounted hydration system compatible with that saddle.

I did a bit of research and came accross the X-Lab Flat Wing and Flat Wing System but I do not want to spend $125 for two X-Lab water bottle cages. Can the Flat Wing accomodate cages other than the X-Lab Gorilla and are there other Selle SMP compatible rear mounted hydration systems out there? 

Thanks!