I cut my extruded uprights to 215mm for both the center and outboard supports (plus one extra). I then marked a line 20mm from each end to line up with the holes that are 175mm apart. Next I came up with an idea for keeping the center supports in rigid alignment while locating and drilling them. I took some 1/2 plywood and trimmed & sanded it so that it would hold the outside edges 70mm apart. I routed the bottom edge of both sides to provide clearance for the inside radius of the extrusion. Finally, I marked a line along the length of each upright 10mm from each edge to line up with the 50mm spacing of the rivet holes. When I clamped them together, laid them down on the web and then flipped the assembly over, the four marks aligned perfectly. I'm travelling to Chicage in just a few hours, so I wasn't able to complete my drilling today, but it should be a snap. This appears to be far easier than the method I used the last time I made my center spar. If I make enough of these things, I'll get really good at it!! No thanks.
