Four key parts of the firewall:
1) flange for front of nacelle
2) smaller flange that glues to the back of #1 to give nacelle planking something to hold on to. Made of light ply and exactly 1/8" smaller than #1 all the way around
3) 1/4" aircraft ply firewall
4) 1/8" aircraft ply cowl ring. Bolts on to firewall to hold cowl on and allow easy accessibilty