Here is one solution for protecting a checked-in rucksack, walking poles and sharp objects, or anything else you cannot take into the cabin.
Last year, I walked my first Camino with an Osprey Kestrel 48 rucksack. I bought and used the Osprey "Airporter" duffel sack to protect the rucksack when checked. As I also had hiking poles and a knife, this was the better method. I used my Sea to Summit 20-liter Ultra Sil day pack as my carry-on. After arrival this day bag rides clipped to my harness for immediate use as a shopping bag, trash pick-up sack, laundry bag, or a city bag when the rucksack is inappropriate.
However, since returning home, I discovered that the fully-loaded 48 liter Kestrel rucksack fits nicely into an XXL sized, heavy duty, Hefty-brand Ziploc clothing storage bag. These very thick über-large plastic bags are available in the U.S. at grocery stores, Wal-Mart, Target, etc. They measure 2.0 feet (24 inches) wide x 2.7 feet (33 inches) tall. The quoted volume is 20 U.S. gallons, or about 75 liters.
There is a handle on the Ziploc bag. But the handle will not carry the weight of the loaded rucksack. On the other hand, the bag weighs only 3.8 ox (107 gm) when empty. Here is the web page with everything you might want to know about these bags:
http://www.ziploc.com/Products/Pages/BigBags.aspx?SizeName=XXL
My thought would be to use one Hefty Ziploc bag - TSA will like it as they can see what is in it - instead of the much heavier duffel bag - any duffel bag. I would seal the "zip seam" with duck tape, and use duck tape to reinforce the handle. This way, the bad might snag, but it can be easily repaired with duck tape (which you should be carrying anyway).
The Ziploc XXL sack folds and stores very compactly. You could either use it as a rucksack liner to keep stuff dry, a seat for damp places, or anything else you could think of to use the mother of all Ziploc bags for. The XXL bags come three to a box.
On arrival, I shift the carry-on items appropriately, collapse the day pack unless I will need it enroute to my starting point, shift the collapsed poles to the outside of the rucksack and collapse the Ziploc bag. Remember how light the Ziploc shipping bag is?
If you not traipsing about Europe and are there solely to do the Camino, at the first post office in Spain, you might mail the Ziploc bag and anything you decided in the walk over the Pyrenees that you will no need until Santiago, to Ivar Rekves' office in Santiago. I usually send things like house keys, my flying compression socks, and depending on weather, any additional travel clothing I have. From Pamplona, you can send about 2 - 3 kg of stuff to Santiago for about 10 Euros. The post office "Correos" has boxes for sale.
I hope this helps.
Happy New Year to all!