I just got myself a new toy from our favourite Cupertino fruit company, a iPhone 6s Plus. This is not about that, but something I have found myself doing countless times since I got it on Monday. I often find myself out on the street or somewhere else where I don't have a table or a flat surface to write on. My solution since I got my first order of Field Notes have been to use the back side of my phone. The 4s and 5s was usable, but far from the best.
The 6s Plus is a different animal. It works perfectly as a writing surface.
If you often find yourself using your lap or something when you need to scribble something down in your Field Notes, try your phone, it's much better, and the larger phones is much better ; I would even go as far as perfect.