Working closely with Cervélo and their wind tunnel testing results, XLAB developed an aerodynamic single rear system specific to Ritchey clamp seat posts with a 10 mm horizontal rear hole.
If the saddle is in the rearward position, you may want to look at the DELTA SONIC XL

Aerodynamic Design
The DELTA SONIC allows the bottle to be close to the saddle, hiding in the rider’s draft and provides clean air flow.

Reliable & Customizable
The DELTA SONIC fits snug into the seat post providing a sturdy non-sway bottle mount, for fast and safe access to your bottle. The DELTA SONIC rod can slide along the seat post for the best customized fit for you. XLAB recommends a 1" insertion minimum into the seatpost.

Compatibility
XLAB has found these bikes to be compatible with the DELTA SONIC.
Cervelo P5 & 2013 P3, Scott Plasma, Kestrel 4000, Fuji D6 & Norcom, BH Aero

Optional Accessories for Cervélo P5, 2013 Cervélo P3 and BH Aero
Pair the DELTA SONIC with the aerodynamic STEALTH POCKET 200 or STEALTH POCKET 300. Bags mount to the top tube on the 2 bolt bosses. Capacity of 20.4 cu.in, these bags can hold the longest energy bars plus some gels and salt tablets.

Includes
DELTA SONIC Rod
GORILLA CAGE
All hardware and installation instructions

Material
Rod: Stainless Steel.
Cage: Carbon Fiber.

Warranty
Rod: 3 Year Warranty.
Cage: Lifetime Warranty.