How Do You Build a Good Boat for “Sink or Float” Activity?

My younger brother, in 2nd grade, is doing a "Sink or Float" activity/ competition where he must build a 6" x 6" boat ( basically, a structure that can float; doesn't need to be shaped or look like one) and small tiles will be placed on it, with the boat that can hold the most being the winner. Does anyone know what kind of structure and materials would be the best to support the most ammount of weight? Some restrictions are that you can't use pre-made materials (like a milk jug, or tupperware) and it HAS to be the 6" x 6" dimension, or it won't fit in the bucket. We constructed a small raft with straws wrapped in tin foil that worked fairly well, and tried to use toilet paper roll tubes to make a "pontoon style" floatation device, but they got soggy. Any ideas are welcome. Thanks.