I am currently working my way through adding stiffeners to rear ribs 1-4. Below are a few pics of how I am doing this.

I made a gage strip for laying out the end locations of the stiffeners. That is seen clamped to the edge in the first pic. Small 90 degree vee blocks are handy for transferring lines around edges. Another handy item I recently purchased(and should have done sooner) are the orange soft jaws for the vise. For what needs to be done here, having the ribs held up vertically is easier than laying down on the table.

The second pic shows drilling in progress. The magnets allow me to hold the stiffeners on and adjust the position. The mini clamp is to keep things from moving while I drill the first few holes. The cleco pallet is close by with the cleco pliers hooked onto the next cleco to be used.