I am working on several fronts at a time right now so my updates might be a little disjointed. There are other door latches and other ways to do this, but this seemed to work well for me on my last build so I am doing it again. No sense re-inventing the wheel....
This is the lower cowl brace rod......(note the nutplates and camlock receivers are just laying there for example)
I use 4 nutplates for the lower cowl panel and three camlocks with the butterfly (or "wing" ) top to hold the side cowl door down. They are spaced, from the forward end of the brace rod at 2,9,16,23 inches and the door camlocks are at 3, 12,1/2 and 22. My brace rod is 25 inches long. Yours may vary a little depending on your engine, mount, nosebowl etc. Right now just drill the #40 hole and do not install the nutplates and camlock receivers just yet. This will allow us to use a silver cleco to hold the paper patterns and the sheetmetal in place while we trim to fit and all that. You will need a nutplate on the forward end of the rod to attach the nosebowl (seen in the far left of the photo above) and the rear attach point will be a hole in the rod with the nutplate on the angle brace attached to the firewall. Like this....
Note - it looks like I need to clean up the back side of the drilled holes. By the way these holes in the brace rods are much easier to drill in your drill press rather than with a hand held.
This is the front nut plate.....
You will want to have the cowl brace rod nutplates as AC Spruce # K1100-08. They are recessed/dimpled so the hardware can fit flush. The screws that hold the lower panel on and the nosebowl on must be flush so the top part (door or cowling) will lay flat. So the nutplates must have a slight dimple to allow for this. Your hardware will include a recessed nutplate, countersunk washer and machine screw like this.......
The recessed washers are AC Spruce # 04-00397 for #6 screws -which mate to a #6 nutplate like K1100-0
04-00392 for #8 screws -which mate to a #8 nutplate like K1100-08
04-00398 for #10
And the Camlock #'s are
You will need AC Spruce part #'s 212-12N receptacles
And " " 2600-2W cad plated wing studs
This is the spacing I used for the top rail. Yours may vary a little as mentioned but hopefully this will give you an idea of what to shoot for.
As long as we are working in this area. This will be the rod that will hold the side door Up/open when checking the oil, doing maint etc. It is made from the same material as your fuel line, (5052 .375 X .035) Flatten the end about an inch in the vice, drill the 3/16 hole, and leave it long for now. We will cut it to fit later.
Hope this helps
Bill