I did a test today. Two stringers of 15x15mm size and one of 20x15mm, fastened into slots cut in 15 mm 5-ply pine as a frame. The 20x15mm stringer was fastened with the 20mm side facing outwards and the 15mm being the depth. I fastened this one and one of the others with 55mm long decking screws with a predrilled hole of 3mm. The other 15x15mm stringer was lashed with two shallow grooves made with a round file to put the cord below the surface so it would not cause a bump in the cloth. With the "frame" resting on and sticking up from concrete floor and the stringers in the air, I then hit each stringer about 15 cm from the join with about the same force with a sledgehammer, imitating hitting a rock hard, increasing the force with each blow until they broke. With about the same force applied both screwed stringers broke across the stringer right at the screw hole and the lashed one broke with also the same force, but it split along the grain from one of the grooves. There was no way I could pull the screws out by yanking on the stringers. The stringers disintegrated first and one screw bent. Lessons: grooves weaken the stringer; long decking screw holds well in 15mm plywood but the hole weakens the stringer.
Idea: A hole in the stringer for the lashing oriented parallel with the surface of the skin and placed in the middle would be in the unstressed (neither compressed or tensed) part of the wood. Drill an 8mm hole in the frame in from the stringer to pass the cord through. Drill a 2 or 3mm hole in the middle of the stringer on each side of the frame about 10mm away from the frame so the cord wraps are off the surface of the frame to easily wrap cord around the wraps to tighten them. Pass the cord through the holes using a large blunt upholstery needle. I'll test this method.
Peter