I have a pair of metal battery boxes in the trunk bolted through the frame rails with 1/2" bolts. Weight in the form of shot bags or iron bars are added or removed as needed. I also have a 25 lb barbell weight bolted where the spare tire goes. Fuel cell is on a "bridge" frame above the wheelwell. I downsized from a 10 gal. to a 5 gal. because I figured using extra fuel as ballast wasn't a good idea.