Originally Posted by
maddmaxx
That sir is a "Cage Rocket". You use it to carry stuff in an unused water bottle cage. It is padded inside and has a cover that opens.
+1
Very handy for commuting. I don't need 2 water bottles for my commute. The Cage Rocket carries my spare inner tube, a multitool, and a tire iron.
I don't use a seat bag on my commuter because in the winter I mount a Dinotte tail light to the seat post.