[Walt Fricke]

When I needed some pretty hefty weight I had a local welding shop cut some standard width half inch steel plate to lengths I specified (I ran it crosswise across the floor pan). He also drilled half inch holes which lined up with each other so I could stack it. Worked just fine as long as I needed it, and they now provide a hard surface on parts of my work bench.

Be sure to use something stout (something better than standard body washers) as backing for your fasteners on the underside of the floor pan. Best, in fact, to weld them onto that side of the floor pan, which will greatly reduce the ability of a 10G flop on your car's top to pull the weights through the sheet metal of the floor pan.