I picked up one of those granite gear original stowaway seat packs a few years ago, just ended up hanging it from the thwart in front of me. It is 18*9*4.5, and can easily hold a couple Plano 3700 boxes with room to spare. I like it because it is quick to go from the pack to clipping it in the boat when you need it, and just as quick to put away if desired when traveling.