Storage or bag on road bike
I saw one a few weeks ago (on someones bike) that attached to the seat post and extended backwards. It mounted on the seat post with a screw on clamp but the bag (and 95% of the weight associated with the bag) could be removed via a clip.
I don’t need anything very large. The bag I saw was just big enough for a t shirt or your lunch.
Many saddlebags work with the German KLICKfix system. The KLICKfix system is a series of brackets including one that goes under the seat and another that goes on the seatpost. In my experience the one that goes on the seat rails is more durable and less wobbly. It also seems to be the one preferred by manufacturers of bags.
One interesting op timberlands tion are the bags made by Blackburn. They have a lifetime warranty on them a quirk of the Blackburn company. Get one and it won’t be throwaway.
Here is one they do for the Klickfix bracket, the Zayante large. It has an expanding main compartment, little pocket for valuables, loop for putting the light on, reflective trim and so on:
I believe such bags are bigger than your current requirements, however, I mention that for others finding this question. There are smaller ‘real saddlebags’ and there are also brackets to go with them if you don’t have saddlebag loops.
Another ‘out there’ solution you might want to look into is a handlebar bag mounted behind the seat. Klickfix adaptors are available for the handleb timberlands ar or the seatpost and you can put both brackets on if you want for marginal weight penalty. When you want to do a bit of map reading, eating, using the camera and so on, you can put the bag on the front, when you want timberlands lighte timberlands r steering and better aero then you can put it on the back.
In summary, the German ‘Klickfix’ brackets are brilliant, easy to use, well constructed and used by many different bag manufacturers. The under seat bracket for ‘saddle stuffer’ bags is very good, for your requirements I would suggest having a look at what bags you can get that use them.