Stay with sand bags or kitty litter, in the spare tire well or in the side wells, stay away from big hard things in floating around the trunk unless they're bolted in.
The best traction devices are decent tires. my 0.02

-~-~-~-~-~-~-current fleet-~-~-~-~-~-~-~-~-~-~-~-~-~-
1986 190D +300,000 miles, 17" Rims, Remus, 575 Watts
1990 Chev Van
1976 Buick Estate (hot rod)
1976 BMW R60/6 bought new