I am building a Guillimot and have had three tries at constructing the strongback out of 2X4s as suggested. I was able to keep them straight by clamping the two lengths together for a couple of weeks before fabrication. But I could never get them not to twist excessivly. So finally I tried using metal studs available at your local building supply. These items are available in 8 and 10 foot lengths and in standard(thin) and heavy duty (thicker) gages. They have a cross section which is U shaped with a little lip on each edge. I found that if you cut off the little lip on one of the edges you could join two of them together to form a 2X4 shaped hollow beam. You can join them together with sheet metal screws and use a short length of wood 2X4 as an internal form to keep the dimensions right. I joined two of these hollow sections together with a wood 2X4 section about 18 inches long. The end forms can also be attched to wood 2X4s and inserted into the hollow ends. The big advantage is these hollow steel beams are straight,and if they are not straight you can bend them to straightness and they will stay there. The disadvantage is that attaching the frame sections is a bit more difficult. You have to use predrilled holes and sheet metal or dry wall screws to attach them. If you are having trouble getting a wooden strongback to be straight and true you might like to try this idea.