Originally Posted by
rodar y rodar
I think that`s one of those Topeak quick connect seat bags. They come in various sizes and look pretty cool:
http://www.topeak.com/products/Bags
What kind of rack did you build? Any pics? I`ve done several different kinds. Here`s one I have "under construction" now. It`s going to be a mini front rack- rando style.

Indeed, it is the small Dynapack.
I use it to carry my rain-clothes so I have them with me all the time