Adding Weight or ballast in the trunk.

33IMP said
I use a big old craftsman open toolbox, bolted thru the floor, and one frame rail. Two feet long, one foot wide, one foot deep, i know i can get almost a hundred pounds in it. Take a padlock off, open the lid, add or subtract whatever.

View attachment 1715744974
Put it over the rear axle to just add weight. Farthest back you can put it on the passenger side, to help traction.
Edit, ignore the splash of sunlight, its not in the trunk at the moment.

I'm pretty sure the rule book gets pretty specific on the box construction and a "by-the-book" tech guy wouldn't allow it. The two little flip latches likely wouldn't contain the ballast in a violent crash and the wall thickness is probably also be a factor in containing the contents as well as keeping it secured through the floor and frame.