Preparing for a bikepacking trip to Cuba, my partner said: 'Ideally, I’d like a bag that’s long and just wide enough to take the cooking pot. Otherwise the material bunches up and it looks ugly.
I made two bags and have posted the instructions to make any tubular bag with a base. The same technique can be used to make a dry bag.
http://www.farawayvisions.com/diy-dry-bag/