I know you already ordered your sling bag from REI (a reasonable choice) but here is something that others might find appropriate. I did.
http://www.zpacks.com/accessories/backpack_lid.shtml The bag is made of an ultra-lightweight, water proof (but not immersible) material.
The attachment points allow it to be fitted to ANY rucksack. Mine (in grey) is fitted to my Osprey Talon 48L rucksack (green and grey). The upper attachment fixes to the sewn, upper mount for the shoulder straps. I used thick (@ 4 mm - 1/8") grey bungee women's hair scrungie bands (the tiny bungee bands women use to pull hair back) to create lower attachment points to the lower shoulder straps. The bag quickly removes from the chest position by way of four side snaps - the same as a sternum belt, but smaller 5/8 inches I think. For a quick stop, simply open the two snaps on one side. The pack remains attached.
The straps provided allow you to attach the bag to the waist belt if you chose. Every bag is different. Some might use a carabiner, or simply tie it on with 550 cord or a short nylon shoelace. But, I was going for a lightweight, functional, color-coordinated chest bag. My solution works for me. The ZPacks chest pack is very flexible in that regard.
Also, the included removable 3/4" - 20 mm shoulder strap converts the bag to a shoulder bag or a waist bag / bum bag / fanny pack.
Oh, and by the way, the ZPacks Chest rig weighs VIRTUALLY NOTHING...LOL! But truly, it is extremely lightweight. I weighed it. The straps and buckles weigh more than the bag.
On a related and very important note, I learned from several New Zealand members of my April-May 2013 "Camino Family" that their AARN rucksacks are superior to most others in that they redistribute enough weight from the rucksack; your back, shoulders and hips to the front of your torso to change the overall geometry of your body while walking. It sounds counter-intuitive and it took me several days of experimenting to work it out.
However, when I shifted my 4 - 500 ml water bottles on my special inox (stainless) clips to the chest strap and waist belt from the side rucksack pockets two things happened. First, I could actually access my water bottles without help or removing the rucksaack; and second, moving a combined just 2 kg of weight (about 4 pounds) it made a HUGE difference in overall rucksack comfort and resultant back or shoulder issues by the end of a day.
BTW, the special stainless (inox) clips I used will convert ANY standard 500 ml, 1 or 1.5 liter bottle to a recyclable "Camino" water bottle. They can be obtained here:
http://www.niteize.com/product/Drink-N-Clip.asp This firm makes all kinds of lightweight gadgets that may be helpful on Camino. I also used their tiny LED torch with the bendy stick handle and it worked great in alburges at night. Many outdoors shops carry some of the NiteIze product line, but I have not yet found a shop that carried all of it.
Oh, I added a silicone, plumbing "O" ring to the neck of the water bottle and holding the clip in place against the bottle to make certain the clip remains in place no matter what - period - and it works. After trial and error, I learned that the #18 sized O-Ring worked best for both U.S. and European sized bottle necks. This O-Ring measures 1 3/16" O.D. x 15/16" I.D. (to you metric folks that is 30 mm outer diameter x 24 mm inner diameter).
I hope this helps.
Buen Camino
Tom (from Virginia)