Well after trying to fit the edge support yesterday, I realized that the floor isn't flat outboard of the rightmost (or leftmost) stiffener. Thus, the measurements I made won't work. I decided to scrap that edge support and start anew.

First thing in order to get some good measurements was to trim the edge of the sub floor panel off so that it was being neither pressed down nor lifted by the longeron. Also, I'm no longer using my wood spacer blocks outboard of the rightmost stiffener. Instead I've just got the spacer blocks just outboard of the center tunnel and have a weight on the panel to keep it from tipping downward. Once set up this way, I was able to get some good measurements for how much my edge support strips need to step down.

A bit of calculating later and I had the measurements I needed.

The forward edge support strip is kind of tricky since at the forward end, it doesn't step down at all. Actually it should step up about 1/32 inch, but I figured that would be nearly impossible for me to do, so I decided to make it flat (lower right corner of this image).

The forward strip didn't fit perfectly at first, but after some tweaking with the brake, seaming pliers, and some good-old bending by hand, it fit reasonably well.

...pretty much the same story for the aft support strip. Looks good!

So, after that I decided to cut the panels for the back of the firewall. I measured twice and confirmed my first measurements of 10 1/2 x 10 5/8. Again I used the router to make these cuts.

WTF?!?! How did I manage to do this?!?! The panel's not wide enough. I'm at a complete loss. I actually measured this twice to be sure I wasn't going to screw this up. Well, as it turns out, it's supposed to be 15 1/2 wide. Fortunately I have enough material to make these over.

Here's the front panel cut to rough shape. I need to notch out the corners to clear the weldments as well as the angles where the center motor mount bolts attach.

It's hard to see, but I'm following a faint pencil line for this notchout. I made this cut just using a chainsaw file. Works great!

A little cleanup with a sanding drum (held by hand...not in the drill press) resulted in a nice, smooth curve.

Two more notching operations completed and this panel is ready for a test fit.