First I cut a sheet of 1/2" plywood into eight 6" wide strips for the two eight foot long halves. Then I cut a half sheet into slightly less than 6" wide strips for the inner beam to join the two eight foot sections.
Glue and screw them up.
Slide the four foot section halfway inside the two eight foot sections (leave 10" between them so that you end up with a beam 16' 10" long).
I put the strongback onto 4 sets of 20" tall legs:
