On our last cruise, on our sailboat, we just bungie corded our regular size propane tank to the stern railing near the gas grill. Yeah, it would get a little corrosion, but we kept doing the exchange for a new filled tank as we went down the coast anyway.
By the time we got back from the Bahamas, though, where we just had to keep getting it refiled, it was getting pretty nasty looking, rust and corrosion wise.
I haven't done it yet, but I have an old large fender holder, that a standard grill propane tank fits in perfectly. I keep meaning to mount that on a stern railing on our present boat, to put the tank it. Right now, at the marina, we just use the green bottles.