If you’ve got expensive items in your backpack then an option to keep your good safe is the Pacsafe 85 Security Web Backpack & Bag Protector. Featuring a patented eXomesh® Ultimate security technology, the PacSafe is an adjustable high-tensile stainless steel locking device, designed to cover and protect a variety of bags and packs from tampering, pilfering and theft.
The mesh goes around your bag, it fits around most packs of different sizes, a loop then secures the opening tight and then can be locked up with the padlock to a secure post / bed / bike stand. The mesh is slash proof, can secure two bags together, folds into 17 x 10 x 5cm and in total weighs 580g including padlock.
If your going to be going away for a while, have one or two expensive items then a Pacsafe 85 Security Web Backpack & Bag Protector could end up saving you on more than one occasion. However, some people feel by using a Pacsafe 85 you draw attention to your pack, which is true which is why I would recommend using in combination with a rain cover, the Pacsafe 85 goes under the rain cover which stops attentions being draw but you still have the security of the Pacsafe 85.