Got the center channel drilled and clecoed today. Need to cut two L angles, 50MM each and drill pilot holes in one side. Clamp behind the center channel and drill through the center channel pilot holes and cleco, then drill through the pilot holes you made and drill through the rib and cleco. Normally I don't step the holes (I use sharp drill bits and find this mostly unnecessary) but in this case, given the tight quarters in which you have to drill, I did go from 3/32 to 1/8 to #20. I used the angle drill but this would also be a good location to use a 12" bit. The edges of the two angles need to be tapered to make sure the skin doesn't touch the edges.
