Agree about just drilling and thought the same thing: put the front dowel of the rail extension into rear of the shorty front rail, get it about straight, then dimple the roof with the mounting stud and drill the hole for the rear mounting stud in the dimple.

However, the rails have a permanent fat "washer" that is a little offset from the stud, and I am wondering whether the drill guide is similarly offset.

Again, the dimensions of a drill guide from someone who actually has one (these guides seem to be exceedingly rare -- rarer than the extensions themselves) would allow me to make one myself on the drill press using the appropriate aluminum stock.