I cut out and glassed the lower firewall mostly per plans as well as the parts for the two rectangular sides.
TIP: I say mostly per plans because I did enlarge the 1"x1" aluminum engine mounting hard-points to 1.5"x1.5". Several builders have reported not having much margin to account for motor mount tolerances so enlarging the mounting locations provide a little more margin.
TIP: Wait to cut out the longeron cutouts or the electrical conduit holes in the firewall until after you've established the true positions during Ch 6. using the fake firewall. You can be somewhat loose cutting the longeron holes in the fake firewall until you get everything line up the way you want it. Once situated, I glued popsicle sticks on the fake firewall around the edge of the longerons to form template I could use to mark a starting point for the cutouts in the real firewall. Of course some minor adjustments were made as I fitted the real firewall but it worked out pretty well.