Once you've marked the center line, you have to match-drill the brace into the fiberglass all the way through to the exterior. The plans are kind of scary here, warning you to ensure you drill perpendicular to the parts. Since the cabin cover is so thick in this area, even a small variation can make the four holes look like they were marked off on the exterior by a drunk person. No one wants that! I ended up doing pretty good I think... not perfect, but pretty good.
Once the four holes were drilled, we removed the cover from the fuselage and began cleaning out all the dust and chips. I fabricated four small tubes as called out in the plans to insert into these four holes we just made in the top. The idea is that these tubes reinforce the attachment point and allow the screws to grip a little better. They also look pretty cool after countersinking them into place:
The four F-10106 spacers before countersinking - don't spend an hour like I did making four spacers EXACTLY to the correct length per the plans... you're just going to whittle them down anyway when countersinking. Just make the interior end nice and leave the exterior end rough.
After countersinking-- looks pretty neat!
Word of advice to future builders: be careful when countersinking these! While running the drill with the countersink, it can grab the spacer and begin spinning the whole thing inside the cabin cover. If your finger is on the other side holding the spacer flush, the spacer basically acts like a mini hole saw and you'll end up with a small, perfectly circular piece of skin that used to be part of your fingertip. Ask me how I know :-)
Next on the agenda is match-drilling the backing strips, but we'll save that for another day... stay tuned!