Another thing I did was lay 1/4" fan fold foam on top of the cross ribs and poured the foam on top of that. Like you said, it eliminates some structural support but I don't use my boats as hard as some so it wasn't a concern. By doing it that way, the water has somewhere to go rather into the foam and if I pick up the front of the boat the water should run to the back. Search rigid pour foam on ebay, 10 gallon kit is 495 with shipping, 40 cubic feet. That should float 2400-2600 pounds.