Not done a canoe, but on yaks it is a 2X4 beam. 1 1/2 inch ply fiting into 4 inch rabeted ply.

Thanks for the reply. The parts that I got are enough to make 4 eight foot long sides. There are two sides of 8" wide boards with a 1/2" rabbet on each edge. One side with an 8" board with square edges, and one side with a 7" board with square edges. I can't figure out if I am supposed to cut the 7 1/2" board into pieces and glue them inside the "U" made by the other three sides or not. That would only make a beam closed on three sides, and the rebbets on the ends of the open side would be of no use unless I cut another piece of 1/2" ply 8" wide to go on that side. Any ideas?

I am not sure about the numbers.  The ones I have built have been 4" wide boards with rabbets on both edges.  Then there were 1-1/2" wide peices with no rabbets.  I then glued/stapled the 1-1/2" peices into the rabbets to make a 2X4 square beam.  The forms should have 2x4 holes in them to slide over the beam.  Make sure that the beam is not too large to allow the forms to easily slide onto the beam.

If I understand your dimensions correctly, you might need to do some ripping/rabbeting to make this work.

If you need to make a beam longer than 10' (which I don't think you do) just make sure you stagger any joints in the beam.