You will need some sort of strongback as shown
on the drawing, on which to build. After using a number of different
systems over the years, I have come to prefer using a box beam.
This is cheap and easy to make and is much more stable than a
solid timber, which is prone to warp and twist. The drawing shows
a novel beam made with two 4"or 6" boards and corrugated
cardboard sides (shear web). The corrugated is glued on with white
glue that is clamped with staples every 3". Be sure that
the corrugations run vertical...no problem to join the corrugated,
glue the butt joints. Again you might feel more comfortable using
the 1/4" luan sides - but they shouldn't be necessary! Shown
below is Platt standing right in the middle of his first box beam.