Regular steel tank "fixed up a bit", and padded.
I use some automotive door gasket glue, made by 3M, from AutoZone, to glue some fuel line that has been cut lengthwise, to the bottom rib of the steel tanks. This has proven to be very durable, and covers up that semi sharp steel rib on the bottom of those steel tanks. Then I made a padded (3 layered) bag. The layers are a heavy nylon canvas, then polar fleece (for padding), then another layer of the heavy nylon canvas. The bag is similar to a heavily padded sleeping bag stuff sack. I made it to use an old NRS raft strap as the drawstring. I just put the tank in the bag, cinch it tightly closed with the strap/draw string thingy. Then I put the whole thing in the bottom of the everything bag. This rides right on the self bailing floor. Keeps the weight very low in the boat. I always figured that the empty steel tank wasn't nearly as heavy as the fuel in the tank. Never had a problem out of this system on 5 years. the guys at freeway propane are always quite impressed with my tank set up. I was too cheap to buy one of the composite tanks, though I must admit the ragasco tanks are pretty trick. Good luck. Hope the above explanation makes sense.