Weight On The Backend...?... Options

[ConcFour]

My Son has a 2007 FX2 , with the Suicide Doors to the back bench seats..Winters in Eastern Canada can make Good Traction a challenge sometimes..He has 4 top of the line Winter Snow Tires ( not All Seasons )...The objective is to add some weight on the back end to improve the traction and doing so without making anything permanent.. Thinking of putting 3 full size Rubbermaid Containers filled with Sand along the Back Bench Seat..My question is , Would this location be to far forward of the Rear Wheels to add sufficient weight to the back end ???