I found a left over piece of 1/4" low density Clark foam as backing material for the aft center section spar that will be exposed to the engine compartment. I followed the Plan and microed the foam pieces in place. Once cured, I trimmed them to conform along the edges of the strakes.
I saved the firewall template from Chapter 6 and used it as a guide to trace out the outline on the aluminum sheet per Plan. Then I lay a translucent paper over the firewall and trace out all the hole locations, such as the engine mount holes, the torque tube holes etc. By the way, you have to remove all the screws and bolts off the firewall to be somewhat precise. I transferred all hole markings onto the aluminum sheet. I cut out the aluminum sheet with my favorite pneumatic grinder/cutter. I followed up with slight sanding to smooth out the sharp edges.
For round holes, I opened them up with a center drill and finished them with the appropriate drill bit. I started with the top engine mount holes first and work all the way down. I tried to be as precise as possible with all the holes, I still have to do some filing and sanding to fit them in. Overall, it fits well.
This is the first time I worked with Fiberfrax, they feel like ultra thick toilet paper. I traced out the outline and holes using the aluminum insulation. Then I cut the Fiberfrax to shape with scissors and razor blades. Unfortunately, I was a bit short of Fiberfrax based on the Plans' dimension.