Tried three different brands, x lab, PD and speedfil single bottle holder behind the saddle. None fit. The rails re just to narrow. Seems such a shame that ISM designed a saddle which does not fit any rear hydration system. Going to try the turbo wing now, but that really seems like overkill for a single bottle.
Is there a reason that zip tiews and a standard cage as stevej
suggested, won't work for you?
I'm pretty sure the PN 3.0 chassis is almost identical to the Attack. I've previously attacked a Specialized Rib Cage to an Attack with zip ties without any problems. It's about the most minimal, lightest, cheapest and lowest drag systems you could use. Unless you need a position this won't accommodate, or you want a frame like a Turbo Wing to mount a repair kit to, there's probably no need to spend more than $20 total.
Incidentally, I've also used an Elite Skekane for rear hydration and that works with the Attack. It's not quite a s tidy a design as the Turbo Wing but does the same job just fine.