Sacks on the floor are inevitable, unless you run like me with never less than a full crew. When we don't have a full crew, usually I put side sacks on either side of the engine compartment and put 2-3 people in the bow to balance the weight. I've thought about trying to plumb something into the floor or under the front seats and there's just not enough room. For all the work, you might get 150-200 lbs if you did both under the floor and under the seats.
Me, I've got a ripped sack in the back from my 4th of July trip. I'm hoping it's just the fitting where the hose enters the sack, but haven't gotten inside there to check it out because I'm sick... it's killing me. If the sack is torn, I'm going to get the Fluid Concepts 1000-pounder and just keep loading more people up front.