check out this site for pre-drilled steel plates - specifically the 12"x12" plates in different thicknesses with 4 holes that could be used for running bolts through; comes in all shapes and sizes. Seems like this could be a good setup for a ballast system utilizing a base plate integrated into the OEM passenger seat location.

http://www.metalplatesource.com/inde...q0h2n2bb8anmv7