Ruffwear Kibble Kaddie
Of all the things I never thought I would end up reviewing this has to be high on the list. But it's actually very good! Essentially it's a durable bag for keeping dog food in whilst on the go. Being durable it means it will last ages, but also that it won't succumb to nibbling to get at the food inside. It has a roll top closure much like a standard dry-bag, which means you can make it whatever size the enclosed food is so it doesn't take up extra space in your sack. There's an elasticated pocket on the outside which neatly fits his collapsible bowl, but most importantly there is a magnetic door which makes pouring the food out super-easy. It holds up to 10l which should be enough for most dogs for a week (that's 42 cups). It retails at £42.95 which seems reasonable for what is essentially a very specialist bit of kit.
You can buy it from Ruffwear.