If I owned that bike and felt the main thing that was missing was more space for inefficient ways to carry stuff, I would get some 1" webbing and tension it between headset and post, then mount a bag that tensioned off that ridgeline, and attached to the top tube. The tension would support the frame and seatpost loads, so it would not hurt anything, and you could stop it sliding down the post with a stiffener. You could then add a bag larger than the size of the one you have there now. Patent pending.

(just noticed the line I am suggesting for the webbing (wide side parallel to the earth) is about the same place as the underside of the wooden stud in the fence behind. Maybe to the rear a little above the level of the toggle on the seat tube)