Carrying a spare on a Cervelo P2 / P3

There have been other threads on this and that is how I found out about this. I got the Xlab pocket rocket bag you can fit like two spare tubes in that thing. I have a Cervelo P2 and it fits perfect on my bike.

I use the XLAB Super Wing and stuff my spare kit in the middle section (between the bottles).

it's not perfect, but it works.

Not sure if you use a rear hydration carrier, but you can get the Sonic Wing and then attach a bag to the rear of it or underneath. You can also have your inflator/CO2s on the Sonic Nut which attaches to the Sonic Wing.

The Rocket Pocket is also a good idea if you don't want a rear carrier.

Agree with above.  In between the seat rails, tucked up near the bottom of the seat to hide it from the wind.

For a HIM, I will carry 2 tubes, so the second tube is in a ziplock bag with my second CO2.  I shove that in my rear tri top pocket as I'm running with my bike.

I put mine this way last weekend when I realized I had no way of easily carrying a spare on my new P2. Does my picture jive with what you're saying?

You mean vertically?  It would rest on the seat post. The other direction, along the top tube axis under the seat it won't fit, there isn't enough space between the seat and the post.

I don't have a picture but my setup for this year for my P3 will be this.

1. Torpedo mounted bottle between the aerobars
2. Bontrager Speed Bottle on the downtube with my flat kit inside of it (since it is basically useless for drinking out of).  I drilled new holes in the cage so that the bottle sits right down in the bottom of the triangle and dremmeled a new hole in the top of the bottle to allow the tube to go in and out easily.  I haven't tested what exactly I can get in there but I suspect I can get two tubes, two C02, levers etc
3. A single bottle mounted directly behind the saddle using a jury rigged/dremmeled Tacx bottle mount (doesn't look anything like that pic - more like this) for when I need more than one bottle of fluids.

I did the taping under the saddle thing last year and it worked well.  I have a Cobb saddle.  I couldn't get the tube completely under the saddle either but I could get the levers, C02 etc under it and then the tube tucked in pretty close and hidden.  I had it lying horizontal rather than vertical like you do in your pic and the bottom of it was higher than the bottom of the seat rails (couldn't see it when looking from the front).