I always thought that the duffle bags were really only for training and where you needed to be able to carry less stuff.
I've a Slazenger V90 Duffle that I got for £9 off ebay. It has enough room inside for kit, a large outer pocket for whites, keys, guff, and a bat pocket. It also has two narrow straps that can be worn as one (but no zip) or a rucksack. Works very well in my opinion!
How much can you carry? Well, with two of the them I carried 6 bats, my training kit and shoes, full protective gear (including Ayrtek), laptop, keys, phones, cameras and sandwiches and had plenty of space to put the super large thigh pad afterwards to the last batoff.