What about one of these for a soft sided captain's box? I got one for Father's Day two years ago.
Hopper 30 Soft Sided Portable Cooler | YETI Coolers
I have been wracking my brain to find a captain's box/bag solution for my boat. I have an adjustable NRS frame. With the aluminum dry box and the captains boxes, the setup looks so pretty. Originally I wanted aluminum captains boxes but can't think how to fit them in my boat without losing the adjustability of the cockpit size. Lately I was thinking about the DownRiver or Stiches and Stuff captains bags. They are soft and not likely to bang shins or tear dry suits. But they're not water tight. The Hopper is water proof and roomy, and has an attachment point right on each end made to pass a cam strap around a crossbar. I know it's spendy but I already own one. Those aluminum captains boxes were pretty spendy too and they wouldn't keep my lunch and beverage cool. I got the 30 liter but now they have a 40 too. The price for two captains boxes made out of aluminum was quoted as over $700-800.
Maybe I'll go see how the one I have rigs up in a boat.