rear firewall

well for mine i just used steel, then used self tapping screws to hold it on through the cross brace and all that, then i put my back in and never had any questions, The track i go to most the time dosnt even notice if your battery isnt in the engine bay or if your running a fuel cell but for a more strict track i would just throw carpet over it and let them look at it through the trunk, they will have a little less to see that way and if theres no light shining through indicating gaping holes through it then you should be ok.
If you make an effort and put something solid there you should be alrite i think but every track is different, im going to run carpet over also so when i remove my seat it will look decent..