exactly what I was looking for. fits perfectly on my 55+15l lowe alpine backpack. tried it also on a 60l backpack and it fits smoothly (there's still room to add that extra pair of shoes if using it for air transportation). if you want to close it with a padlock you must find a very thin one, though!
This is one of the greatest products in mountaineering, and definitely something everyone should have. It covers the entire pack, which is great as a rain cover but more importantly it doesn't allow any handling other than the straps on the top and bottom while transferring (car, plane, etc..). As someone who has lost a pack to poor handling in the airport, I can safely say this cover (or a similar one) is a must have for everyone.
I bought this as I went backpacking for few weeks and I needed to keep my bag safe, especially when being at airports.
PROS: It does the job fairly well by keeping your backpack safe from dirt and allows you to lock the zip (you need a padlock). I also like the fact that you can keep it on even when you carry the backpack on your shoulders.
CONS: It's relatively weak. The very first trip I used it I lifted the backpack -- which weighted 12-13KG -- by the cover handle and the stitching ripped apart making the whole thing pretty much useless. You can imagine how it became after the airport operators handled it.