Cheap roll top dry bag (8-10 liter) and one or two straps should do the trick for under 20 bucks (probably less if you really look). Put stuff you want to access in last so you can just unroll it and grab stuff without actually undoing the straps that hold it down. Other options which could be found at second hand shop: Soft sided childrens' lunchbox (preferably with superheros on it), camera bag, childrens' backpack with straps cut off, soft sided camping cooler of some sort...