FIA Eye bolt backing plate is 3mm thick steel plate, 67mm X 67mm, with a 7/16"UNF nut welded to it that is used as a a load spreader when not utilising OE seat belt mountings.
Being steel it can be welded to the body shell and it is plated for corrossion resistance.
Care should be taken to ensure that any panel used for harness mounting is of sufficient strength & thickness to cope with the forces experienced during an accident.