I am using an 11ga (1/8") steel plate welded in below the fuel cell on my Spec Racer Toyota to protect the fuel cell from puncture. It also protects the fuel pump inside the cell from damage if you were to directly impact it from below.

Prior to installing this plate the fuel cell step and the back firewall have to be removed to trim 1/4" off the bottom of each which takes the rivet flange off the bottom of the step. Next a 1" square tube is welded between the 2 existing frame members and the plate welded after that. This can all be done with the floor still on the car. It is important that the weld beads are smooth and no splatter is left on the floor plate. The panels are installed riveting the step to the side of the new 1" tube.

This is the 11ga plate which weighs 4.6 lb the tube adds another half #.