I've used a box beam about 4"x4" made from staggered, glued, screwed plywood for an external strongback and it held up well, although next time I use an external strongback I'll try Bjorn Thomasson's method.
On Nick's recent series of videos he's using an extruded aluminum beam as an internal strongback, a little more expensive but the ultimate in durability (and Nick uses a sweet system for setting it up).
A friend of mine is building a Petrel using engineered lumber (a 1-1/2"x3-1/2" beam) as an internal strongback. The stuff's relatively inexpensive, dimensionally very stable and looks like it will hold up well for a home builder. I plan to borrow it when I staert building my next boat later this summer.