> I like the way a 2x4 plywood box down the center of the forms looks. It
> appeals to my engineering aesthetic eye from a perpective of the way
> airplanes are sometimes built. It seems elegant and practical

In the interest of accuracy, the strongback pictured is a standard 2x4 instead of a box beam. A 2x4 works great if you can find one which is reasonably straight. Let it sit in your work shop a while before using it to let it acclimate.