Wow, whatever I had really knocked me for a loop, but thanks to modern medicine I am mostly over it at last. Irregularly-scheduled blogging will now (sporadically) resume.
I pulled the forward top skin out of storage and clecoed it to the fuselage:
The cowl on my last airplane was attached with a combination of quarter-turn camlocs and piano hinge, instead of using hinges everywhere as the plans show. That seemed to work all right, so I've decided to use a similar setup on the current airplane project. The bottom cowl will be attached to the firewall with straight sections of piano hinge, and I'll use camlocs to attach the top cowl to the firewall and along the horizontal seam between the top and bottom cowl halves.
The camlocs need a metal backing strip attached to the fuselage, to which the camloc receptacles will be mounted. I cut a 3" strip off the end of a sheet of 0.062" alclad (gee, I really wish I had a big metal shear) and shaped it with hand pressure until it conformed to the curve of the firewall.
While clamping it carefully in place, I match drilled every other hole through the forward edge of the top skin and the firewall flange. If you plan it carefully, you can use a single piece with no compound curves that runs from a point just above the upper longeron all the way over to the corresponding spot on the opposite side of the firewall. Then you add small filler pieces below the firewall bend line, which I'll show in a bit.
To mark a trim line a fixed distance forward of the firewall flange, I drilled a 1/16" hole at a suitable location in a piece of scrap aluminum, stuck a pen through it, and dragged it along the mounting strip. Simple.
Lots of cutting and filing later, I had the mounting strip trimmed down to size.
It's hard to see here, but I've now marked and drilled #40 pilot holes where the camlocs will eventually go. You can also see one of the extra filler pieces that brings the line of fasteners all the way down to the horizontal cowl seam.
The instructions recommend a maximum spacing of 3.5"-4" between adjacent camlocs; however, since I remember seeing the cowl "pillowing" between fasteners at high speed on my last airplane, I decided to reduce the spacing to 3 1/8". That also helped keep the hole pattern away from the split between the main piece and the two smaller pieces.