Since my compact frame Trek has only one bottle cage, I added a seat post mounted carrier so I could carry two extra bottles. This is great for long rides, but the bottle holders didn't leave enough room for an under the seat bag or a blinky light on my seatpost.

Then I found this little bag made by Lezyne that was small enough to fit between the two bottle cages and big enough to carry a spare tube and a few other items. It also has a loop to attach the tail light. The whole setup worked out real well. They (Lezyne) also make a slightly larger bag, the M Caddy, that might also fit.

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Products Used and Links
Profile Design Aqua Rack - http://www.ebikestop.com/profile_des...ack-WC1605.php
Lezyne S Caddy - http://www.lezyne.com/index.php/prod...s-caddy-3.html
Planet Bike Super Blinky - http://ecom1.planetbike.com/3034.html