Jesper:
For my guillemot double I used a 2x4 pine that remained straight thru the 4 month process. Even if it is off a little, the oversized holes will give you some fudge factor. Another possibility I not heard mentioned might to use metal studs used for 2x4 construction. When I finished my basement it sure was nice to have every stud straight as can be. Just stagger the seams as you overlap the butt ends of the 2x4's and screw them together. They have great lateral strength by them selves as a single stud. I'd imagine they would be very strong if you put two together. It would be lighter and might be another option for you to consider.

DAVE