on my wee-lassie knock-off, I used 1 1/2 by 3/4 WRC for the gunwales, with two thwarts, one at the backrest, the other by my feet, as well as breasthooks. The gunwales are mortised for ribs, (as in basic greenland construction, except that the mortises are cut at the outside edge at a 25 degree angle) rather than an inwale/outwale.

The result is plenty stiff for a 12 foot boat.

I don't know what to expect from the plywood sections, but with the rib type construction, the thwarts seem to be in tension.