After my first three Caminos, and after trying just about everything I could find, I determined the "best" solution was a ultra lightweight shoulder bag that could be worn with or without my rucksack. This proved to be an ideal place for items I wanted to reach quickly, but did not want to consign to pockets on my rucksack, or shoulder harness. Remember, this is all offered as an IMHO suggestion. There is no correct solution. In the end, what works best for you...works best for YOU.
Here are my two considered "ideal" solutions:
Sea-to-Summit Sling / Shoulder Bag: www.seatosummit.com/product/?item=Ultra-Sil%26reg%3B+Sling+Bag&o1=0&o2=0&o3=380-36
I buy them on Amazon.com. This bag is made of ultra-sil nylon fabric and is VERY lightweight, as well as water resistant. It compresses to the the size of a Kiwi fruit, yet holds about 16 liters. It also has a zip-closing top flap. It makes the ideal, in the cabin, carry-on for flying to your Camino starting point. It can be stowed in a rucksack pocket, clipped to a belt or harness, or just worn over one shoulder, before you hoist your rucksack. It is also handy as a "possibles bag" for when you need to buy stuff at a tienda, supermercado, or farmacia, on your way into town. The name comes from the use...you might possibly need to use it...to tote stuff you pick up along the way.
ZPacks Multi-Pack - 4 in 1 Backpack Lid / Chest Pack / Belt Pack / Satchel: www.zpacks.com/accessories/backpack_lid.shtml
This bag must be ordered direct from the FL (USA) maker. The fabric is an ultralight and more durable grey Cuben Fiber. While it does not compress as small as the Sea-to-Summit bag, and only holds about three liters in volume, it was my daily "go to" bag for carrying snacks, iPhone, guidebook, and my handy dandy notepad for making notes along the way. I also stowed the compressed, Sea-to-Summit shoulder bag in this sack when I did not need it as a "possibles bag."
The really neat thing about the ZPacks bag is that it can be configured as a shoulder bag, waist bag, attached to the top or back of your rucksack. It comes with a variety of clips, straps and click fasteners to attach it as a belly or chest pack too. Only your imagination limits the attaching possibilities.
I suggest a lightweight shoulder bag, such as these, instead of attaching more stuff to your rucksack. I think you will find the result more to your liking.
For the record, I carried both on my most recent fourth Camino this April, from Madrid, until I had to stop at Sarria due to injuries.
I hope this helps.