M_Boats: Recommended Ballast on M15

Hello I am the owner of 2006 M15. I have a 43 lb Yamaha 4 stroke mounted on port stern. The boat lists from the outboards weight. I would like to add ballast to trim the boat. Its a very tight space under the starboard bench. The boat was built with extra ballast glassed into the bow. What have others used or what might you recommend I use to correct this situation. Sandra Anton Barnegat Bay NJ