I don't own a monty and have no experience with positive flotation. I do have a suggestion as to how to add it however.
      If you opened up a heavy duty plastic bag inside an empty space on the boat and sprayed insulation foam from a can into the bag, The expanding foam would fill the space completely. The plastic bag would keep it from sticking to the hull and make cutting it out easier if needed.
      The plastic bag could be replaced with any container or shape that would suit your purpose such as making a cooler or storage space.

Sandy