I like the box beam method like Matt suggested.
The first strong back I built was for a kayak I designed for myself for small waters (which was a lesson for me to leave the designing to the pros). The strong back was made out of 3 layers of 5/8 inch particle board glued and screwed together. Its very dimensionally stable. It was eleven feet long by 1 7/8” x 4”. It also weighed close to 5000 pounds. At least that is what it felt like. Positives: it was nice and straight and didn't sag. Negatives: Heavy. I still have it in case I ever decide to build another smaller boat. I don't have the heart to bust it up and trash it.
I recently built a 17 foot strong back out of 1/2 inch plywood in the form of a box beam using glue and staples to hold it together. This was for a canoe for which I have had the forms for many years, and finally decided to use them. Its for an 18 1/2 footer that will be used for my family of 5. After starting the stripping last summer and being nowhere near done, I wish I would have bought the Sasquatch 18-5 . It has very similar lines. But, I had already invested in the materials several years ago and had all the forms, so I figured I may as well use them. I'm one of those people who has to finish something once they start it. Especially if there was a financial investment in it. Besides, I needed to get the materials out of the garage so I can build my Sabalo.
Here is the cross section of a piece I cut off. You can see I missed a couple of staples.
Here is the full length
I pounded a nail into the center of the SB on each end and tightened a piece of braided fishing line between to make a perfectly straight centerline and to be sure there was no curvature to the SB. Then, I lined the centerline of each form up on this string line being careful not to move it. Its not a good photo at all. I also screwed the supporting structures to the floor of my back deck after lining everything up so things can't get bumped out of line by my two boys.
It will bend side to side some over the 17 foot length, but doesn't sag. I think it would be hard to find any material that wouldn't flex over that length. I also supported it in 5 different points over the 17 feet. It is much lighter than solid wood. Particle board in particular.
Of course the canoe will be on the back burner once I get the Sabalo plans I ordered.
Sorry, If got too detailed and hijacked your thread, I didn't mean to go off on a tangent.