Hello, this year I am planning on trying to ride 2 centuries per month, but sometimes there is so much stuff to bring along it makes the back of a jersey sag. I have a very small underseat pack with a tube and c02 and multi tool, but do not have much room for more. The bag I am considering seems like it would be perfect for my centuries and other really long rides. What do you think about this bag? Do you think I can mount it on a carbon seatpost? http://www.topeak.com/products/Bags/DynaPack THANKS!