One of the most common and inexpensive water resistent backpacks is this model (also comes in black and navy blue). $25 US from China of course (eBay). It fits all models of Phantoms as long as you modify it. In the second photo you can see my 3 modifications. 1 - Made space for P3S Controller. 2 - Made a hole the size of props. I can fit 8 props in here. 3 - Spray painted one battery slot so that I quickly know where I've placed the spent battery. I've used this here in Canada in the snow and it keeps the Phantom dry.