Relocate batteries to reduce tongue weight

I would like to reduce the tongue weight of our 2009 WW FB2100. There are currently 2 6v batteries mounted on a rack on the tongue. There is enough depth at the rear to mount them under the bed of the trailer in the center near the spare tire. I can make access plates in the bed of the trailer for 2 drop in battery boxes. The boxes would be vented underneath and I know the requirements for cable sizing. I've been searching for an in-floor mount/drop-in style battery box, but haven't found much. Has anyone seen anything like that? In a few weeks I'm going to set up a lever scale to measure the tongue weight and shift the batteries to the rear for a second measure before cutting any holes. I'm thinking moving the batteries from 11ft in front of the wheels to 4ft behind has got to have some effect. It would be a more secure location for the batteries and still be accessible.