The plans tell you to "slip the W-412 tip rib into place", but with the wingtip clecoed to the wing there's no obvious way to manipulate the rib once it's inside. I solved this with a couple strips of duct tape:
Once I was happy with the position of the rib, I marked and drilled the rivet holes in the top and bottom:
The flanges on the tip ribs interfere with the nutplate reinforcing strips I glued to the inside of the wingtip flanges, which I suspected would happen. I guess I could have planned that better, but it's no matter, as I actually came up with an improvement while dealing with this. First I relieved the rib flanges to fix the interference:
I replaced the cut-off rib flanges with some pieces of aluminum angle that sit on top of the wingtip flanges and their reinforcing strips. This will end up being stronger than the stock design, since forces acting on the tip rib will be transferred to the wing structure via the three aftmost screw holes:
Finished ribs after riveting the modified flanges:
I countersunk the rivet holes and then attached the ribs to the wingtips. To avoid cracking the thin fiberglass, I erred on the side of slightly under-squeezing these rivets. Here you can also see how the add-on rib flanges sit on top of the reinforcing strips:
One of many test-fitting sessions to make sure everything is lined up:
I'm happy enough with the way the wingtip ribs turned out. I could have done a better job of planning ahead, but I'd probably use the same basic approach again. I think this is going to be stronger than having a rib that's supported by nothing but flimsy fiberglass.