Adding strips to the forms will stiffen the "system" considerably.

Supporting the strongback at its "quarter points" (1/4 its length and 3/4 its length) will keep it from sagging in the middle or drooping at the ends.

Using drywall screws and plenty of glue will help make the strongback very dimensionally-stable. Nails don't cut it. Screws alone will do pretty well, but not as tight as with glue.

Like Joe and Rob said, use the stringline for "straight", not the top surface of your strongback.

Shawn

: I can only tell of my one experience, others with more experience may know
: more about it. I used the same box beam external strong back design
: (5/8" ply). After assembly and setting it up on 2 horses and
: supports, I noticed it soon was not perfectly straight. I allowed it too
: settle (2 weeks) and resupported it. I was able to be then certain it was
: stable and would not settle any more. Then to my mind as long as it would
: not change from that point IT WAS FINE TO USE. I just used it as a solid
: platform and set all the forms straight upon it using only the string line
: as the guide. Then any "bends" of the strongback had no effect
: and if it no longer moves it is fine. I did this and my boat is straight
: as an arrow. Good luck.