Consider getting the serfas that I suggested. It will work right out of the box, is lighter, rechargeable, and substantially brighter. Put your superflash on your helmet.
As mentioned above- the Serfas can be mounted on a P3/P2 or most any aero seatpost without having to go through the process of cutting PVC and retrofitting an aero post into a round post. It works fine just by itself. I also tried a small cut piece of foam pipe insulation- as sold at any hardware store, and it makes the aero post a bit rounder and even snugger fit.
Also, if you have a decorated Christmas tree hanging off your seatpost/seat- the light can be mounted upside down- towards the brakes and still tilted appropriately towards the eyes of drivers in cars AND/OR it can potentially be mounted where your seatstay becomes one piece above the rear brake. I can mount it there nicely on a size 54cm S3 Cervelo aero road bike.
5 star average review at amazon - http://www.amazon.com/...eywords=serfas+usl60
light and foam together
light on cervelo P3 bike with small piece of foam insulation helping to round the post
light on cervelo S3 size 48cm - an a 54 cm S3, I can nicely mount the light on the seatstay, just above the rear brake bolt.
serfas thunderbolt option for P3 without any foam pipe backing (it also goes nicely on many seatstays- for additional light).