With your saddle being that close to the tire, I think a total re-think might be warranted.
The racks that clamp onto a seatpost (I assume your seatpost is not carbon) can hold a rack top bag quite nicely and they are not that heavy. Looks like your saddle is pushed back on the post pretty far, but still a rack top bag might be the best option.
I attached a photo of my road bike with the type of rack and bag I am talking about.
