sounds ok if you can get a small enough peice to push, but big enough to flot it 
You can take two slim peices of plastic or wood, and put one on top, and one on the bottom at the rear of the boat/sled put four holes through both, then sandwich the pieces with the foam in the middle.You should be able to build any type of mount you want on top of the flat piece.
If the foam you are using has a hard pladtic bottom, then bolt whatever you like right through the foam and plastic.
You can take two slim peices of plastic or wood, and put one on top, and one on the bottom at the rear of the boat/sled put four holes through both, then sandwich the pieces with the foam in the middle.You should be able to build any type of mount you want on top of the flat piece.
If the foam you are using has a hard pladtic bottom, then bolt whatever you like right through the foam and plastic.